If Bom has 300 seeds, then Gwi has 300+40 = <<300+40=340>>340 watermelon seeds.
Together, Bom and Gwi have 340+300 = <<340+300=640>>640 melon seeds.
Yeon's number of watermelon seeds is three times as many as the number that Gwi has, meaning Yeon has 3*340 = <<3*340=1020>>1020 watermelon seeds.
Together, the three friends have 1020+640 = <<1020+640=1660>>1660 watermelon seeds.
#### 1660