yes a cold can cause weight gain (water weight)...at least in my experience. Are you taking meds for the cold? That could do it as well. Have you just started weight lifting and walking? Muscles that are being worked tend to hold onto more water which could easily be your 3 lbs and then some. I have read lots of times on here where starting an exercise regimen will cause a gain and then level itself out. Just keep going - as long as your doing what your supposed to the scale will eventually fall in line.

Most frustrating for me is the moon - I swear I gain weight during different tides lol.