I honestly don't think that Yoshi has a slight disadvantage to Marth. If we are being super nice, I feel it's 50:50 (even but difficult), but 50:50 (even) seems nearer the truth. In turn, I'd also put Lucina in 55:45.
Yoshi's relatively good frame data is enough to keep the Marth matchup even. While his Nair can pose a threat to Marth up close, luckily, Marth definitely has the aerial move advantage. Marth however must be wary of Yoshi's aerial mobility, as Yoshi can suddenly be upon him due to his speed. But then Yoshi must also be wary because he lacks a solid safe way to deal with Marth's disjoint, so he must respect it. The thing with this matchup, is that Yoshi has tools that he can use to pick Marth apart, but actually getting in there to do so can pose some trouble, due to Marth's range and quick ground normal, and his good aerial moves.
The thing about Lucina, is that she lacks a tipper, and I feel this is a matchup where it can hinder her where Marth has it as a noticeable advantage. Basically, unless she lands a raw Fsmash, it can be relatively tough to land a KO. Sure, she can Jab to Fsmash, but that's not true until very high percents, but even then, Yoshi's Super Armor double jump is a "get out of jail free" card, and prevents accidental mistakes. So Jab to Ftilt is the best option here, and Lucina cannot KO off of it until above 150%. Marth, however, can KO with this at around 120%. Another disadvantage to no tipper is the lower knockback. Basically, if Marth hits a tipper in neutral or during footsies at a relatively high percent, it can put Yoshi in a difficult position off stage because of their power. Lucina's blade (or Marth's sourspots) won't hit him far enough to pose enough of a threat until very high percents. This makes her edgeguarding game weaker. Due to the decreased power of her aerials compared to Marth's tippered aerials, Yoshi can sometimes survive a Fair or Bair when Marth hitting with tipper Fair or Bair would KO. Both of these characters lack substantial ways to combo Yoshi due to his easy combo break double jump, so this matchup is generally all about who can punish Yoshi harder, or punish him harder off stage, and Marth has a noticeable advantage in both these areas.
So yeah, that's my thoughts on these matchups. With everything culminated together, I still can't see how Marth has a slight advantage on Yoshi. It's at least somewhere even, and Lucina is slightly below wherever Marth is I feel.