Jeep not starting after sitting for 3 weeks

Squrit some carb cleaner or starting fluid into the intake, if it fires up it's a fuel issue. If not, it's likely electrical or mechanical. Hope this helps a little.
If it fires up on ether, check the fuel pressure at the fuel rail, if no or low, check the fuse (should have a fuse diagram in the fuse compartment) if that's good, replace the pump. If you have pressure, check the injector wireing with a set of noid lights. They should all flash when cranking engine. If all checks out good, check resistance on all injectors, or replace them (pretty expensive, and rare to have to do).