Is a cannondale scalpel 3 full suspension capable of trail/ downhill? (100mm Suspension) I have found myself to be more of a trail rider than an XC rider and would like to get into downhill. I have a cannondale scalpel 3 and I was wondering if I can make this bike work. Possibly buying the new 130mm lefty supermax or other mods. Or just trade in for a new bike.
A lot depends on what you mean by "trail/downhill" and by "work". 

It wasn't that long ago that a full squish 100mm fork bike was a full on downhill machine. However, the big drawback to that bike as a descender is the relatively steep head angles. 

Putting a bigger fork will help with that, but it won't help with the issue of how robust the parts are relative to the use. Lefty forks are meant for XC lightweight
applications, not for landing 10 ft drops. Putting a slightly longer and more robust fork on that bike will help a lot with feeling in control on descents, but it's still an XC bike and has limitations in how hard you can ride it. 

It all depends how rugged your trails are and how hard you intend to ride.  If you still want a mostly XC bike that handles descents more effortlessly then a fork upgrade makes sense. However, if you're looking to go beyond that then you really should consider getting a different bike. 

Having said all that, outside of a terrain park, I don't think there are many trails that couldn't be descended with that bike. It's just a question of how fast, how much fun and how much skill is required.