Theoretically, there are countless possibilities of how you could go about EV training them. But there are some things you should definitely do.
Sceptile should definitely have its sp.attack stat maxed out.
Blaziken should have a lot of investment in both attacking stats, but not necessarily maxed out on both of them.
Swampert should have max HP. From there, you could split the rest into offensive stats, or give it some defensive EVs.
For the remaining EVs on Sceptile and Blaziken, it depends on what you feel is the most important. You could max out speed, but then it's an all-or-nothing situation where if you're outrun, the EVs will have been a waste.
I don't know any generic EV spreads, I just use whatever I feel will work best.