Hi there - just to let you know, the Petsmart in my area (Canada) does free water testing, just bring in a little baggie of water. Hopefully they do this in Illinois too.
It would be smart to ask them what the actual values for nitrite, nitrate, ammonia, and pH are. Sometimes they will tell you your water is fine without showing you the values... because they just want you to buy more fish.
Your values should be:
ammonia: 0
nitrite: 0
nitrate: below 40
pH: between 6 and 8
Values higher than 0 in ammonia and nitrite indicate that your tank is not cycled yet. Sorry if I'm repeating things that have already been said.
Test kits are good if you can afford one eventually because changes in a small tank can be more drastic. Also, the liquid based test kits are more accurate than the strip tests, or so I've heard.
I'd say 2 platies and 1 algae eater are enough for that tank, even overstocked.

You'll need to find another home for the angelfish and gourami. Sorry to be the bearer of bad news. The general rule is one inch in length of fish per gallon. That means with 2 platies at an adult length of at least 2 inches you'll be overstocked in a 3 gallon.
Don't feel too bad about not knowing this stuff beforehand. It's not like Petsmart tells you how to properly take care of fish. I started there too and my tank was a disaster: I killed 2 fish and a snail from ammonia poisoning and disease.

I've learned so much on this forum in just a month and you will too!
Good luck!