Compound Interest... with Wizard Money
R, 70 62 bytes



function(d,i,y)(x=d%*%(a=c(1,29,493))*(1+i)^y)%/%a%%c(29,17,x)


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Takes input as d: deposit in knuts, sickles, galleons; i: interest rate as decimal; y: years. Outputs final deposit in knuts, sickles, galleons. Thanks to @Giuseppe for using matrix multiplication to save some bytes (and pointing out how to avoid the need to wrap at 1e99).