The three weeks falls out in the middle of camp season, and camp trips are often an integral part of the summer program. Rav Vosner was asked (Shevet HaLevi 10:26) if it is proper for camps to take children on trips in the three weeks before the nine days start.
He answered that from Shiva Asar B'Tammuz until Rosh Chodesh Av, "Yesh LiMa'et Ktzas B'Tiulim", the trips should be somewhat minimized in order that the children should recognize that these changes are because these are days of mourning. In the event that trips are made, they should become a Derech Mitzva by discussing Divrei Torah and Chizuk the entire way.
Important Note: We try to convey the Tshuva to the best of our ability. We admit that our understanding may not be accurate. One should learn the tshuva to verify the accuracy of our interpretation. Please also understand that this Tshuva may not be the final word on this topic. One should consult a Rav before drawing any conclusions.