There are several different ways of strumming triplets. Each of them is useful in particular musical situations. The most common way is to use all downstrokes of the pick. Try this method with the following example, which contains a root 6 A7 chord and a root 5 D7 chord, see lesson 35.
Once you can play this example evenly, try using alternate strumming. This means that beats 2 and 4 will begin with an upstroke. To be sure you keep the correct rhythm, count out loud as you play and tap your foot on each beat.
Another useful strumming technique for triplets is to use an upstroke on the middle part of the triplet only, making sure you begin each new beat with a downstroke. Once again, tap your foot on each beat and count out loud as you play.