They can be really hard matchups especially because Chrom and Marth or so fast. You’ve gotta use your shield a lot and try to parry and whiff punish everything they miss. Stay just out of their sword range and rush in with fair or dash attack when they give you an opening. Dash dance and jump around and try to bait out their attacks. Hit them with PK Fire on their landings and whiffs. Once you get them to about 60% try and get them in the air and start juggling them with PK Thunder.
They’re hard matchups but they’re definitely not impossible and you do have some advantages if you use PK Fire and PK Thunder well. Also edgeguard them both hard. Marth can be gimped with the tail of PK Thunder if he tries to recover low and you can hit Chrom with PK Fire on his recovery and spike him out of it (get consistent with Ness’s spike, it’ll help in a lot of matchups and the PK Fire spike set up is legit ). I go about 50/50 on these matches and it’s not the worst matchup in the world just tough because their disjoints outrange yours. That doesn’t mean fair, nair, uair, and bair aren’t good in the matchups though. Also fastfall all your aerials