This ran around the boards years ago regarding fans being to strong and blowing trichs off. Now I guess this could happen if it was super windy, and this depends on what type of fan you are using, but I doubt this is your problem. Now the 3rd week of flowering means that they have pretty much just started to bud. Stretching can last 3 weeks, so give them some time. You are 12/12 right? Do the plants look healthy? Light leaks?