Can I put a red tailed shark and 2 three-spot gourami in a 55 gallon tank?

Short answer: Yes!

Long answer below

Species selection (length, difficulty of care):

• Red tailed shark (4", intermediate) are semi-aggressive.
• Blue gourami (6", easy) are peaceful.
• Hatchets:
  • marble (2", intermediate) are peaceful;
  • marthae (2", intermediate) are peaceful;
  • silver (2", difficult) are peaceful.

Aesthetically speaking, getting a school of fish is probably what you want to do. It allows the tank to still have fish, but it really showcases your tank "stars", the gourami and shark. If you do get schooling fish, you really do want to get a school of them - they do best and are easiest to care for that way.

I might also recommend a catfish (like cory cat or Otocinclus) to help a little with the cleaning.

Semi-Aggressive

Semi-aggressive means under the right conditions the fish can be very aggressive. Personally, I have found this to be common when the water quality is bad or the tank is overstocked. With semi-aggressive, no one can say for sure there will be no problems - only that if everything is good, there shouldn't be any issues.

Tank size

Assuming you get 6 hatchets and according to the "1 inch of fish per 1 gallon of water" rule for tank stocking, that's 4" + 6"∙2 + 2"∙6 = 32" < 55 gallon. You still have plenty of room for more fish and shouldn't have any issues. With a 55 gallon tank, you may even feel your tank looks too empty. :-P

Potential problems

I really think the bulk of your problems are going to come with being new to aquariums. Since you are getting a 55 gallon, you'll avoid a lot of the issues that beginners face with 10 and 20 gallon tanks where the water quality nosedives very fast because there is so little water in the system. Having live plants is a whole new level of complexity and I had a hard time getting the hang of it - I really had to do my reading to understand what was happening and buying the chemicals was a must for healthy plants.

If you find that these fish are too challenging for you at first, you can always try less-demanding fish until you get the hang of it.