I am running two new baseboard heaters on a 20 amp circuit, also 2 outlets on a 20 amp circuit, can I run 12/3 wire and share the same same neutral from a junction box, these are on 2 separate walls in the same room, also one of the breakers will be for the heaters, and the other for the 2 outlet units.
If your heaters are 120V then it's possible to use a shared neutral for the heater circuit and the receptacle circuit, as long as you follow the code rules that apply for this type of circuit. (If the heaters are 240V, there is no neutral on that circuit, and they'd have to be wired separately.)
If you haven't already done so you might want to consider whether you're better off with 240V heaters and separate wiring. Even if I didn't want 240V now, I'd wire these separately so that I had the option of converting to 204V in the future, rather than save a little wire now with the shared neutral.
Yes you can share a neutral on a 120v circuit. This is called a multi wire branch circuit. You will need a double pole breaker or at least a handle tied pair of breakers. This is to keep the power on each circuit on opposite legs. If the breakers ended up on the same leg the neutral would be overloaded and could cause a fire. This is covered in the NEC 210.4