i just wander if i had a death +1 focus, and got three of them and salvaged all 3 of them and got 3 +1 death mod, can i put all three in one wepon? if not why not?
You only have 2 slots for mods on a focus item, a focus core and an inscription slot. Your +1 death mod(which would be the "Master of My Domain" inscription) will take up the sole inscription slot and any attempts to add more will just replace the one that's there already.
As to why one can't just add as many mods to a single item as they want, well, that's not hard to figure out.