1. LR3 you can inject just about any time, the LR3 version lasts longer and is more "systemic" in nature compared to DES. I would go with pre-workout since it gives a nice pump and also it won't interfere as much with natural MGF (Mechano Growth Factor) levels post workout.
2. LR3 will not be as site specific as DES. Some say you don't even have to inject LR3 IM, you can pin sub-Q and it will have a similar effect. If you're using LR3 it would affect all muscles, not just arm. If you use DES, then you would inject the working muscle group bi-laterally pre-workout (i.e., 50mcg each bicep, 100mcg total, if working biceps).