In star trek universe, it is said a drop of red matter can collapse a star. A type III civilization is capable of harnessing energy of the entire galaxy, therefore can I assume they can develop a weapon that can fire a projectile or beam at a distance and create a temporary singularity which will consume everything in its vicinity? if so will it be portable or a structure mounted?
I think the question of "portability" hardly comes into play for a type III civilization. I would expect a type III civilization to be easily capable of accelerating entire planets to near light speed (and slowing them near their destination).
OTOH, causing a black hole at a distance may not be possible within the laws of physics. It is more likely that it will be possible to create a black hole within the weapon and launch it towards a target, rather than creating it remotely.
With currently known physics, the only way to create gravity is by mass. So to create a singularity they would have to create a large amount of mass.
There is a theorized method to create mass from energy: photon-photon collision.
However, your requirement was to create a temporary singularity. Any black hole large enough to be weaponized would also be large enough to be stable enough to survive for aeons.