My brother just started playing bass and we have a question. He's right handed by the way. Should he practice using two or three fingers(index, middle and ring) of his right hand? Why?

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You should start out with two fingers, and once you feel comfortable and fast like that, you can move on to three or four fingers if you want to, though there are many people who only use the index and middle fingers.
If he's just starting he should only be using his index and middle finger alternating. Some bassists use three to fingerpick but that is something that you'd only develop after already being very skilled at 2 fingers.
start out with two, when your good with two and feel the need for a 3rd, then use your ring finger too.
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Word. Practice playing with two fingers for a while before attempting three. Try to make it so while you practice, you are always alternating fingers, it might make the eventual transition easier.