The first appears to have trifoliate leaves. I have never seen Trifoliate Orange (Poncirus trifoliata) but maybe that's it?

The yellow is likely some deficiency rather than an identifying feature of the species or cultivar.

What do they smell like? 1st looks like maybe a lemon or citron, 2nd like an orange of some variety - not yet fully ripe, and 3rd like a tangerine/mandarin, also not quite ripe.

First just smells a bit off... second def not an orange. It gets yellow spots but will fall off the tree before turning full yellow. It is also more the size of lime not bigger. Third is probably a tangerine, first time i see it giving fruit so i will wait and see if it ripens.

Have you tasted them yet?

Trifoliate Orange has been used as a root stock plant. The grafted plant may have died, and left the rootstock to take over.

Number 1 has trifoliate leaves so it isn't Lemon or Citron. I don't know anything much about Citrus but I do know that Poncirus is used as a root stock as gyr_falcon said.

Could you post pictures of the entire plants?

# 1 is not trifoliate orange (Poncirus trifoliata) as the leaves are too wide, but is for sure a trifoliate hybrid such as citrange, citrumelo or similar. It is used as a rootstock. In your case, this rootstock overwhelmed the plant and killed off the scion (edible variety)...as gyr mentioned. So it's basically worthless, unless you have the know-how to graft back upon it.

Some actually do squeeze the juice of these hybrids, and depending on the actual hybrid, add lots of sugar. But one must be careful to avoid sqeezing the peel as it has a lot of off, oily poncirus flavors.

Here are some other pics of plant 1. Will post a pic of the whole tree in a bit

might location matter???

there used to be a citrus forum ... might find some other peeps there...

ken

Location northern california

Would guess #1 is Citrange.