I would say the 3100 for $115-120 is perfectly fine for a low-budget system. For Intel, the equivalent would be the 10100 in pricing, though the 10100 has better gaming performance even when you overclock the 3100 (cant OC the 10100). Obviously the 3300x for the same price would be better, but as you said, can't find it.
The 3100 is slightly better for production tasks, like Adobe Premiere Pro, so if it's for any productivity, get the 3100. If it's for gaming, get the 10100. If it's for normal overall usage, I would get the 3100, mostly since you can overclock even on the budget boards if they upgrade the CPU later.