@shaheenmissile
Hi dear you mean higher Isp fuel? the solid fuel has two components as you are aware of,
1)the oxidizer NH4ClO4(ammonium perchlorate) although RDX and HMX have also been reported to be used in american trident d5
2)and the fuel HTBP that also acts as binder.
The trick to increase Isp is to replace perchlorate with more explosive mixtures such as HMX etc but that leads to itz own problems of instability either combustion or acoustic.
Now coming to the question of increased range of shaheen-3,since there aint any structural modifications in the missile,i guess they would have
1)either reduced the weight of missile by replacing maraging steel with composites- this option is highly unlikely given the amount of research required in composites and requirement of sophisticated winding machines
2)second option could be they would have tweaked the propellant by substituting perchlorate with something else or by adding powdered aluminium