Using the RESULT of a roll in a macro

So, my friends and I play with a variant on DnD 3.5 that the crit damage is a direct multiplication of the roll instead of rolling again x-1 times, where x is the crit multiplier. My question is:
Is it possible to write something that would able me to multiply the result of a roll instead of rerolling it x-1 times?
i.e.
What I want: (assuming the result was a critical roll)

If 4d6+3=15   then a crit dmg (X3)would show     crit dmg = 45 since its just picked the RESULT of the roll and made the multiplication as in 15*3 or RESULT*3

What I got: (assuming the result was a critical roll)

If 4d6+3 = 15   then a crit dmg (X3)would show    crit dmg = (well, anything between 7 and 27)*3

I know I could just create separate macros for attack rolls, then one for normal damage and another for crit damage. I want to know if its possible to create all in one single macro so the player just hit one button and then math tells him if it was a crit or not and if it was the crit dmg using this rule of ours.

Nope. You can't rely on 'memory' of something to do a calculation. You need to do it all at once or not at all UNLESS you use API.

PSA: The API is a Pro subscriber perk.

ok thanks for your help.