Possible? Sure, at the far end of the Kardashev scale a lot of things might be possible. Practical and useful though? I doubt it.
The problem with weaponizing sheer gravitational mass in this way is that this is about one of the lowest energy thing you can do with a given amount of mass. Consider a mass of one ton. It has gravity that is so small you can't meaningfully measure it due to the inconvenient presence of a whole universe around it. However if you roll it down a hill it'll destroy an armored vehicle. Drop it from a plane and it'll destroy a main battle tank. Fire it from a cannon and it'll smash a building. Make it out of high explosives and it'll level a block. Make it out of fissile material and it'll level a city. And those are just things we could do with one ton today. Antimatter or just relativistic speeds open up whole other worlds of destructive potential for our modest ton.
To get any kind of meaningful time dilation you'd be talking about very dense stellar mass objects - things like neutron stars and black holes.
That's a lot of mass to collect in one place for a weapon, and very inconvenient to move once it's created, so it becomes more of a navigational hazard than a weapon.
If you have that much mass to build with you can do a lot of other destructive things with it. You could make enough high explosives to devastate numerous worlds. Or nuclear weapons. Or antimatter weapons. You could just separate it out into quintillions of 1 kilogram masses and fire them at a modest 0.1 c and each one would have nearly the destructive potential of a 100 kiloton nuclear weapon. At .9c that is into the megatons.
Something else you could do to create a weaponized black hole is a kugelblitz. In principle you can create a black hole with energy rather than mass... But as with mass this is a positively staggering amount of energy. You could just point a tiny fraction of that energy at the ship and vaporize it instead.
The second idea, crushing a target between two other ships, falls into the same problem. A space ship can exert far more force with its engines than it can from gravity - rather than using gravity to crush that ship the other two ships could press it between them while burning at full throttle and crush it. Or just ram the target at high speeds and destroy it in a much faster and more thorough way.
There is a reason why most of the history of weapons has involved figuring out ways for small things to hit big things faster and father away than the other guy's small things can hit your big things.
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