The Dose for SafeGaurd Liquid Dewormer is 0.23ml per pound, so just under a 1/4cc, this is given orally, once a day for five days. Also, a 3ml syringe is pretty hard to give it with, you'd be much better off using a 1ml syringe that's slimmer and will readily fit into their mouth better.
It's fine if you just want to just eliminate the possibility of it being Gapeworm, but it should be known that Gapeworm is pretty rare with chickens, a respiratory disease, for example, would be more likely.