S4lad :
Can I use DDR3 DIMM in DDR3L/DDR3 Slots in a motherboard, on gigabytes website it says that it supports both + the cpu i want uses DDR4/DDR3L
¿Could?, yes but with a potential risk on the long term.
It's safe? No, the problem is that the voltage conversor is within the CPU chip, and it's limited to 1,35v for DDR3L (witha built in 10% safety margin). You could use an 1.5v voltage and it will (normally) not burn instantly but it's no safe on the long term.
If you could undervolt the DDR3 then it becomes (slightly) more safe, and if your DDR3 ask for more than 1.5 (for example they are 1.65v modules) then the answer is directly a no go.