insert three numbers between 3 and 19 such that the resulting sequence is an ap

let the no. be a2, a3 $ a4. it is given that first no a=3 $ a5=19. We know that an=a+(n-1)d. Therefore, a5=a+(5-1)d or 19=3+4*d 16=4d or d=4. now a2=a+d=3+4=7. similarly, a3=11 $ a4=15. Therefore, the required no are: 7, 11, and 15