It really depends on how big they are and how your body reacts to different substances. Perhaps the answer is 3ml and perhaps it will be 0ml.
Personally when ever I shoot in bi's, tri's, delt's I have never gone above 1ml.
I always used the glutes and quads as main injection sites.
I know a few people who inject 2-3mls in delts but in honesty, no one can really answer this question other than with a generic answer.
Remember it also depends on the substance, I would never inject suspension or prop into my delts, well not anymore.