Three consecutive A.P. terms: (a-d,\ a,\ a+d), sum (3a=21\implies a=7)

After subtracting 1,4,3: [ 6-d,\ 3,\ 4+d ] in G.P.

[ 9=(6-d)(4+d)=24+6d-4d-d^2=24+2d-d^2 ] [ d^2-2d-15=0\implies(d-5)(d+3)=0 ] [ d=5\text{ or }-3 ]

Numbers: (2,7,12) or (10,7,4)

Answer: (2,7,12) or (10,7,4)