Falcon: Nair destroys him, just try not to jump into his own nair or trade with any of his other aerials. Dash out of the way to avoid OoS options as necessary, or utilt. If he starts doing more ground moves, stay out of their range more or usmash OoS (depending on how close you can get). CC first hit of nair->whatever. WD OoS if you ever feel uncomfortable. Usmash attempts to space nair. If he ever attempts to approach with an actual attack, he's pretty much always going to lose to or trade with sh bair, dash dance and good shield game are his ways around it. Overshoot approaches to deal with DD unless he's pivot nairing. Utilt and shine make him have a rotten day. Edgeguard with bair for high recoveries, and don't be afraid to double jump. Grab ledge or fsmash for lower recoveries or ones where he might go on stage or to the ledge. Dsmash at the ledge if his up-b is from below. DJ drill from the ledge if he's below but you're unsure if you can get the shine. His OoS is significantly worse behind, exploit this.
Sheik: Grab or shine her, don't get grabbed. Try to avoid going for uthrow uair at 0. Most Sheik's love ftilt utilt and dsmash, as they should, so punish the first directly out of shield on shield DI in with grounded options or dair towards, make no attempt to punish utilt if the second hit hits your shield, and wavedash in at the end of dsmash->shine, or otherwise bait these out with movement. Avoid needles, don't be too hasty in recovery, most Sheiks prey on that. If possible, DI ftilt out, or at higher percents, SDI ftilt and dtilt up. If you get grabbed, it's better to try to fight your way out. Either try to shine, or some other fast option depending on what Sheik does if she hasn't grabbed you.
If Sheik's roughly at the edge of her dash attack range, and doing what looks like a dash attack at all, feel free to assume it's a dash attack as even if she does a max distance boost grab, it hits later and doesn't go as far (and most Sheiks probably will dash attack anyway for some reason). Punish any rising aerial of any kind OoS.
Sheik has a similar problem with OoS options from behind as Falcon, but is less likely to try to retaliate from behind (though the options that hit there are actually faster). If you're slightly farther behind you can dash forward a little, if close enough dash attack any kind of jump (reserve for higher percents), or keep running/dash back to cover and kind of escape options. For spot dodge (very distinct sound and animation) you can sh drill in place -> shine or grab (do not fast fall). Drill typically trades with or beats all ground options, but some of these trades are unfavorable, and it's unsafe on shield and vulnerable to SDI.
To edgeguard, pay attention to if Sheik has a double jump, and try not to go deep. Vanish isn't invincible until frame 18, so if you don't see her start it yet, and she's in range for it, you should usually be able to rising bair or drop down and shine. If Sheik can sweetspot the ledge from the ascent of Vanish or her doublejump and you are not at the ledge, it is not the time to grab it (from stage or refreshing). In the case of the Vanish, do so a little bit earlier, and for the double jump, even earlier before. Regular get up and get up attack (under 100) are preferable to ledgeroll, and it's more preferable not to have to do either. For higher recoveries, try to intercept Sheik on the way down from a Vanish, not the landing. Try to catch Sheik doing anything before she lands with a bair one of your strong grounded anti-airs. Don't run into Sheik at max range for one of her aerials if she's at the ledge or a platform (or you're on a platform).
Fox: Don't get grabbed or shined. Don't jump into utilt usmash or shine. Try to avoid challenging pressure if it's close, or DI out and usmash. Don't get hit when near the ledge, particularly by shine. Block bair, or DD around it (like any attack really). You can shoot lasers in this mu. Edgeguards will consist of a lot of bair, you can dip low and do rising drill over and over against lower Fire Fox usage. Anything you want to avoid getting caught by, you want to catch the opponent with. Nair is CCable to surprisingly high percents. Illusion from higher heights can generally be caught with a bair. If it's too high for you to comfortably bair, you can wait for it to come down, lag doesn't get lower until heights higher than Yoshi's top platform. If it's closer to the ledge or at, you can hold ledge, waveland on to punish, time a get up attack, or dip lower and come up with a rising bair from the ledge. If onstage, jab, ftilt, utilt, dsmash or usmash are all fast and relatively easy to land (though they may get past certain options, pay attention to height). Shine isn't even too hard to hit, but if you're not confident about positioning or timing for anything, just shield and WD OoS. To beat shorten and no shorten, you can ledgecancel bair->ledgegrab, shine->wavedash back onto ledge, or jab twice. If at ledge height, but not enough to go over, either hold edge, ftilt down, drill, or dsmash. Try to avoid attempting to directly punish crossups on shield, and if you do them yourself, dash dancing should beat bair OoS (pay attention to when you hit and where you land).
Falco: Don't get shined (so watch out for CC), or grabbed, really. To get around lasers, CC them, act out of shield quickly, powershield, use platforms, or hit Falco out of the startup if possible. Get hit by lasers on throws and SDI away from Falco. You're faster than Falco, so if you deal with lasers well, maintaing good spacing isn't too hard. Edgeguarding isn't too different from Fox, but he doesn't have any flames to worry about during Fire Bird, and his recovery moves come out a little faster (and hurt worse if they hit you), and have worse range. Illusion into lasers to gain recovery distance. Uthrow destroys, and shining him near the edge puts him in really, really bad places. Don't challenge the bird when he's above, don't run into utilt or AC bair walls. Utilt is worse as a KO move from him, but combos better. It's generally hard to hit him at lower percents outside of a punish or grab because of CC shine potentially meaning you lose a stock.