If all of the gravel being used has been stored in the bucket that formerly held soap, then I would not use it but that's just me.
If the bucket that formerly held soap was rinsed really well before placing the gravel in ,then perhaps it would pose less risk.
If you don't know whether the bucket was rinsed well, then I might give all of the gravel several rinsings and place it in a clean container or as mentioned,, Get some new gravel.
At just three cups,, I am doubtful that filters,hoses,decorations ,would be too affected but don't take my word on this. Soap residue depending on the amount,can be quite difficult to clear from the tank.
I keep gravel in old five gallon buckets that once held floor wax but I use nearly scalding hot water to rinse them out before using them.
Use same buckets for acclimating new fish.
I might purchase a couple clean buckets from Walmart to use for my tanks were it me.They aren't very expensive and I would keep them somewhere where family members were'nt likely to grab one for uses that would harm my fishes. I use a black marker and write... "For fish tanks only " on mine.