Just to reassure you, three fairly small fancies is a very small bioload for a 55g, Since you are adding the old (unwashed) gravel from a 5g UGF the new filter is now seeded with at least a minimal bacterial colony, if you add no more fish for a couple of weeks and do not clean the new filter in that time, and do not over feed, your new colony will easily catch up to the bioload before any ammonia spike can occur. Your tank will start cycling smoothly in an almost unnoticible fashion. No worries. The gradual introduction of fish into an oversize tank avoids the fish killing results of an uncycled tank.
As an aside, it is common usage now days for people to speak of having "cycled" their tank, this is a misnomer as it implies that the cycle is now over, it is not. The cycle refers to bacteria reducing urea and ammonnia to nitrites, then nitrates. This happens continuously in a healthy tank. More properly the term should be "started an ammonia reducing cycle", as long as your fish produce waste the cycle will continue. What the term cycling actually refers to is allowing your bacterial colony to rewach parity with the waste produced. Where people run into problems is when they allow impatience to over rule allowing for bacterial growth.
Under current usage of the term I do not cycle my tanks, even so, it has been decades since I lost a fish to "new tank syndrome" or ammonia poisoning.
Sorry about the rant, it is just a pet peeve of mine and has been ever since the "need for proper cycling" has bled over into FW fishkeeping from SW.
All that being said, for large fish in relatively small tanks, pre-cycling is required as the bioload starts out so high that the ammonia levels rise faster than the bacteria can catch up with, the same thing happens when you fully stock a large tank with new filters, with a little patience and only gradually approaching your target stocking level you can avoid the whole scenario.