would that give me a static proxy IP ? or a dynamic one?
WinGate uses your existing internet connection, so you will have to ask your ISP if your IP address is static or dynamic.
would it give me 3 to use at any time in any session?
A 3 user license allows 3 client computers to connect through your WinGate service concurrently. There are no session restrictions for connected computers.
would HTTP-POSTing ability be possible with it.. So I could for example post of a forum.
The WinGate proxy supports POST.
How long is it for. Is the $75 an unlimited time duration, or just a one year thing or what?
WinGate licenses are perpetual, which means that it will last the lifetime of the product. WinGate does come with one year of free version protection, after which you can choose to renew version protection, or let it lapse. Note that it's cheaper to renew version protection before it lapses if you plan to keep it up to date.