If you do have the hardened, jelly like, layer on the top when you open them, do as suggested and remove that first. THEN, since you have multiple cans I would suggest you mix it ALL together into a larger container like a 1 gallon can and stir or shake it thoroughly.
The reason I suggest this is because if the paint has started to harden or cure in the can you can be assured it was at least somewhat seperated when it began to cure in the can. When that happens it is doubtful that all three cans will have been seperated the exact same amount and when they each hardened accross the top surface they surely did not do so the same in each can. The result is that there MAY be slight color variations in them now. By mixing them all together thoroughly in one container you will be sure to have ONE consistant color.
Also, as mentioned, be sure you filter it. Any place that sells automotive paints will have disposable strainer type funnels you can pour it through when mixing it up in the gun to spray it.