D.C.
This may or may not work for you.
When my oldest was 8 and his younger brother was 3, they would sleep peaceably with each other if the younger could feel his older brother beside him (just a touch once in a while). I don't know if you have room to line up the two beds beside each other. I wonder if that would help.
Does the 3-yr-old nap during the day? When I cut back on the naps, I could get my young one to sleep easier at night.
One other thing, lately, I would bring my laptop to my two younger boys (now 7-yr-old and 10-yr-old) and sit there in the (almost) dark and work. There would be a couple of times (okay, 3 or 4 times) that I would simply say "It's time for being quiet, hush now" or "lay back down, it's time for being still." Seems like they both fell asleep faster.
You will probably get some other, and probably better, advice soon.
Good luck! I admire any single mom who manages to get through this type of phase. Remember: "this too shall pass"