Don't use 3cc of liquid. 3cc is the maximum amount of fluid used for an IM injection (and shots with that much fluid go into the hip or butt), and since most people here use insulin syringes to inject K, you're not even 100% sure that it's all going to end up in deep muscle tissue.
How much are you planning on converting back to liquid? Most bottles are 100mg/ml, and I doubt if you'll be doing more than that so you won't need to use more than 1cc.
I re-converted powder for injecting a week ago. I used water from a small sterile injecting vial, but I work in a hospital so I have access to shit like that. You can go to a drug store, however, and buy a big bottle of sterile saline or sterile water; just ask the pharm person for help and tell them you need some for a relative that requires wound irrigation. They'll get you what you need. And if after you attempt to dissolve it you see ANYTHING but clear fluid in the syringe, don't do it.