I am a Junior in high school and have one SAT subject test down, Biology M, which I took freshman year. I would like 2 or 3 ideally when applying next year. Which tests are the easiest to take and which should I take? I was thinking US history considering I am taking APUSH right now. Pertaining as to what classes I have taken, it is mostly consistent of the normal schedule (aka chem, physics, bio and us, euro, etc.
Definitely take US History, and do it in May if you can. That’s what my D did–her test dates were a day apart. You will have already studied for it, why not maximize your time investment? D found the subject test easy after the APUSH test.
Beyond that, take whichever subjects interest you. 
I completely agree with @ShrimpBurrito! Definitely take US History if you’re taking APUSH. If any of your other classes are AP then take those subject tests (if possible) too! If you’re in AP Chem or AP Physics C (or AP Physics 1&2) take the chemistry and physics subject tests if those interest you. For physics, make sure you have both mechanics and E&M (plus a few other random topics that seem like they’d be in ap physics 2) down and don’t take it while still doing physics C mechanics w/out any E&M knowledge (that’s what I did…bad decision). Also, get a review book and look at the practice questions on the SAT subject test website–they’re really helpful!
Also, if you’re looking for any easy test, Math II is pretty easy and you can use pretty much any calculator. You don’t even have to get all the questions right to get an 800 and don’t need any knowledge of calc. It may be better to leave off math until the beginning of senior year (then you’ll have pre-calc out of the way) unless you’re taking calc now as a junior, then take it whenever (just before most colleges need test scores).
Speaking of college, do you know what colleges you’re interested in? If you know a couple you think you might apply to next year check out their policies on SAT Subject tests (i.e. are they required? If so, how many? Which subjects?). Some colleges require a you take a Math, some require a 2 of any subject, some don’t require any and say that they’re either recommended or optional for example. You may end up not having to take any at all! But either way, even if they’re optional, a good score can’t really hurt your application (unless the college specifically tells you not to send any SAT subject test scores).