To start with your ps3 it may or may not be downgrade able. With that model it is hard to say. Did you buy it new if you did what firmware came on it when you bought. Some ps3s that came shipped with 3.56 or higher. It can't be downgraded, then easiest way to check is there is a program out there that you can install on ps3 and it will tell how low you can go on downgrade. I think it is called minverchecker.
Ok now for dual boot. The e3 does do the dual boot but it onlys works with certain boards. Check the e3 website and see if your board is supported. There is a guide that will help with this on their website.
visiting mod--60 phat,320 & 160 progskeet installed soldered cfw,320 & 160 slim ofw 4.21,120 slim 3.41 cfw teensy 2.0++ installed soldered, 40 phat nor cfw