You can manage 3 fantails in a 30 gal without too much work once it gets established.
In the beginning, though, you would need to do a bit of monitoring & water changes until the cycle is established. If you are not familiar with the nitrogen cycle, I would encourage you to read the cycling article in the article section.
In a cold water tank, it will take 6-12 weeks to establish the cycle. To maintain fish health, it is imperative that you maintain good water parameters during the process with water changes. You would need to do daily water testing to track how the tank is progressing, so a single test is not enough (and you also need to know the levels ... not just "fine" or "safe"). To get accurate levels, you would need to use a liquid test kit ... as bluerose suggested.
What are the sizes of the fish? What is your water temperature? How often & how much water are you changing? What are you feeding the fish? Are all 3 fish common metallic fantails? or the 2 smaller ones different breed?
The 45 degree stance is indicative of swimbladder problem, which may be caused by:
1. Poor water quality - this is the first thing to check & correct. In the absence of test results, I would suggest doing 25% water changes (make sure water is dechlorinated & temp matched) daily or every other day.
2. Infection - the white poop may indicate internal bacterial infection. Generally the fish would also be lethargic, and may have clamped fins, or maybe breathing fast, or show other signs (spots, red veins in fins, etc.) Look at the fish carefully & see if anything is wrong either in appearance or behavior. I would not treat for infection at this point unless there are additional signs or symptoms, and NOT before getting water parameters controlled.
3. Gut dysfunction - Constipation may interfere with swimbladder function. This is esp. common if the fish is fed only flakes, or if the temp is very low. It will not hurt anything to feed some peas (blanched, shelled & chopped up) to see if things improved. <In fact, you should feed veggies to your golds often to prevent gut problems.>
4. Congenital swimbladder problem - Some golds are deformed (bent spine) & will always have issues with the swimbladder.