Yeah three is good if you're looking to use them as a recreational stimulant. If you have a prescription you only have a limited supply though, if you do them three at a time frequently your supply wont last. Amphetamines are addictive and tolerence will develop. You might be able to get away with it for a while, but if you do more than the recommended dose often enough you'll find that your regular dose doesn't cut it and doesn't relieve the symptoms to which you were prescribed the drug originally.
If you feel that you have difficulty thinking and concentrating on tasks then this drug, used as directed, may benefit you.
You have to decide whether you're going to this drug to help you or just to get you fucked up.