well bowls do not usually require an additional oxygen pump, but since u have 3 fishes i suggest u add another oxygen source.
one more reason maybe that the water maybe dirty(due to over feeding)which reduces the oxygen content. try doing a 20% water change at an interval of 5 days.
also due to rise in temperatures, the oxygen content drops. try moving to a cooler place.
Fish also breath through their mouth, as in ur case... gasping for air at the top.
all of the above are from my experiences when i had a fish bowl(it eventually broke... bad quality)
I hope it helps. take care