Not sure of the difference between ENV 2 and 3 and the LFOs

Not sure of the difference between ENV 2 and 3 and the LFOs, please teach me :-)

There's a lot of overlap, it isn't much different in Serum or any other synth in this regard, besides the fact that the Serum LFO's can behave a bit like envelopes (retrigger on note on and not loop).  

So for simple things such as modulate a parameter across X amount of time from note start, they can be used somewhat interchangeably.   
LFO's have much more advanced graph editing ability / graph presets / BPM sync.
Envelopes offer simpler control, automation of simple points (e.g. sustain level) and a note release stage.

Basically the more time you spend with Serum and both Env's and LFO's these differences will make more sense and you'll know which one to grab (or either as again for simple things sometimes it isn't a big deal which you choose).