*Living here in the midwest, the sweetest peaches I can come by still pale in comparison to true southern, tree-ripened peaches. If your fruit are especially sweet, you can feel free to start with less sugar, and add more to taste. (Keep in mind that if you plan on canning your jam, less sugar means a shorter shelf life.)
**The heat of a hot pepper is contained (mostly) in the seeds and veins. To control how spicy your jam is, you can leave these in (hot!), or remove some or all of them (mild). I found I really liked the level of heat from de-seeding one of the peppers, while leaving the other two. If you aren't sure how much heat you want, I suggest de-seeding all but one of the peppers, and adjust from there with future batches to find what you like best. (And of course, if you just want some good ol' fashioned peach jam, you can leave the jalapenos out all together. Ain't nothing wrong with that!)
Keep in mind that the jam will mellow considerably over the first 24-48 hours after being made, so don't worry if it seems spicier than you want at first. When I first tasted my batch, it was waaaay spicier than I wanted. The next day? Perfection!