Fingers have to be trained slowly like dogs.
When you start out they simply have no idea where to go and you're trying to teach them a sequence of movements they're not used to. So expect it to be slow and nasty for the first few weeks.
Start by placing the 3 fingers on the right strings. Use your other hand to help position them if necessary. Test out each string to see if it 'rings' properly. Once you're set, have a bit of a bash and strum up and down a bit, just to reward yourself.
Then lift your fingers ever so slightly off the strings and put them back down again. Check all the strings to see if they still sound right (they might not to start with). Keep repeating this until you're getting it clean every time. Then lift the fingers off a bit more. Slowly work your way towards being able to put them down from right off the fretboard or from other positions altogether.
To begin with your first two fingers (index and middle) will be
fairly good at the job, but the ring and pinkie will need plenty of work to build up both strength and the ability before they will be much good at going where you tell them.
It will be a tad boring at first, so while you do the repetitions it might help to also work on a few strum patterns so that the sound doesn't get too montonous.
Good luck. Keep it at it and soon you'll hardly remember how come it was ever hard. Just don't get discouraged when it takes longer than you hoped.
Cheers,
Chris