Is there anywhere in your area like a nursery or Extension office that you could bring pics to and some of those leaves and maybe a branch or twig or two....?

Looks like leaf miners and possibly the need for minerals. Fertilizer too if you have not done that.

Also, looks like one of the trunks has a good set of leaves and the other two not so good. I would evaluate the the trunks at the base and make sure one or more are not suckers.

Does the entire tree give you lemons? Are they the same?

the harvest looks fine; i have none right now. does this effect the yield.