I recently purchased a small 1.5 gallon aquarium set for my 6 yr old son along w/ 3 guppies. This is the first fishtank we've owned and I figured it would be a good start for my son to learn to take care of pets. The set included almost everything needed for startup such as the filter, gravel, food and some artificial plants.
The third day we had the tank setup, one of the guppies got sucked up through the filter and had a fin damaged. He could no longer swim and we soon had to flush him away. We noticed the remaining fish liked to swim right under the pump intake and became worried this may happen again.
The following day, after replacing the deceased guppy, one of the others sure enough got sucked up through the filter again, and we had to dispose of another incapacitated fish.
As you may imagine, I am running out of patience and do not want any more fish harmed. Please, does anyone have any ideas or can tell me what I'm doing wrong? If it matters, the type of filter is a Tetra Whisper 3i. Thanks for any help.

I don't know about models of pump, but it's probably too powerful - get a smaller one or put some kind of mesh around the inlet to prevent the fish from getting too close.

There are several good specialist websites about fishkeeping - take a look at their FAQ sections or ask in the forums.

Your filter, is the water flow through the pump intake tube powered by bubbles? If so, the bottom of the tube should have came with a plastic end with small slits for the water and debris to flow though. This piece is suppose to keep fish from being sucked up.
Is yours missing this piece? Or, are the fish fitting through the slits?
Either way, you can do what I do for my breeding/baby tank so that babies do not get sucked up the filter.
I cut a piece of net from an old fish net and placed it around the water inlet tube, then I secured it with a rubber band. This is a good way to keep fish out, but it doesn't allow debris through either.
I took a pic so you can get a better idea of what I'm talking about...
2705
If yours is missing the piece, you may be able to find a replacement piece.
You could also take it back to the store and exchange it for one that has all the necessary pieces.
Sometimes when a piece is missing, the store will take the missing piece from another box and give it to you as well.
Good luck
Kae