For starters, one Arrowana will require a 300g gallon tank of 8 feet long and at least 3 feet wide, so if you have 2, do the math. You can't put a fish that grows 3 feet iin a 3 foot long tank and expect it to behave normally.
2nd, 3 days is not a big deal for a fish to refuse food considering it takes at least a week for them to acclimate to a new tank. A healthy fish will not starve itself to death. Give it another week or so.
3rd, since you are a beginner and don't seem to know much about fishkeeping I recommend taking the Arrowanas back to the pet store and starting out with something basic like Tetras, Danios, Guppys, Mollys, Convicts, etc. Arrowanas are for highly experienced keepers only, not people new to the hobby.