Betty,
There are no rules when you are cross stitching. You could use 2 or 3 strands--make a couple of stitches and see what looks better to you. Be sure you have enough thread using 2 strands. Happy stitching!

Like maubennet said -- it totally depends on you and the look you want to achieve. It's also going to depend on the fabric that you're using. If the 'squares' of the fabric are fairly large - 11 or 14 per inch, you might want to use 3 strands of floss for good coverage. If the squares are smaller - 16 or 18 per inch, you might want to use 2 strands of floss because it's really tricky to get the three strands of floss in and out of the same hole as you're making the stitches.