Odontocidium Tiger Crow underwatering?
 

I got this plant a few weeks ago. It's growing roots and new shoots, but some of the leaves are yellowing. A couple already fell off. I know oncidiums tend to lose leaves on old bulbs, but this doesn't seem like a leaf that should be dropping.

It seems to be potted in medium bark and perlite. I water every three days usually.

It gets the same amount of light as my Sharry Baby, next to it, which is currently spiking. I potted that in fine bark (the smallest Orchiata bark).

Am I underwatering or overwatering?

I'm not sure, but I would thing you're underwatering. In medium bark you can probably water every day and not harm the plant.

Could it be too hot?

Hm, I make sure that the bottom half (at least) of the pot never really dries out though. Would they need more water than that?

Here it never really goes above 30C, and I doubt it gets warmer than 26C in my kitchen where this is growing.

It's very difficult to tell from that photo – is the failing leaf part of the new growth, or from the older one to the forefront?

It could simply be normal leaf loss and nothing to be concerned about. These Oncidiinae hybrids start out with a tight cluster of leaves at the base, and gradually lose those lower leaves as pseudobulbs develop.

Be careful not to get water in between the leaves if you are indoors. Sometimes fungus infections can cause those symptoms.

It's from the older one. I figure that the newest old pseudobulb will keep three leaves, one on top and two on the sides, so this falling off might be abnormal?