Perhaps you would have to shorten the chain a bit? I am not too mechanical so I won't say for sure.
if you are not changing the 2 big cogs, don't shorten the chain,
though It's often a good idea to replace the chain , preemptive maintenance.
if its a 74 BCD 3rd cog , 24 is all you have,
a Mountain bike compact crank with a 94/58 BCD lets the gear size drop to a 20t