I do not make all these house rules because I think C&C is broken. I think C&C's Siege engine is great. But I see no reason not to tweak the system a little. I'm a tinkerer for one' and second I like to mold the system to the setting. I dont always run C&C in the same setting.
Along with the rest of my huge list of house rules for C&C. Why not have a 3d6 option.
I really do like a bell curve. But using 3d6 does mean adjusting a few numbers. One great thing about the siege engine is that most all rolls are against a base difficulty of 12 or 18, depending on if the attribute you are rolling against is prime or not. So all that really needs adjusted in those base target numbers.
First lets looks at primary attributes. A primary attribute base is 12 in standard C&C. For a d20 this is this is supposed to be a roll that is just slightly easier than 50% for a 1st level character with an above average prime attribute.. For a 3d6 most rolls will be average of 10.I think this would be a good target number for a primary attribute roll.
For secondary attributes the base difficulty is 18.
This would be way too difficult for a 3d6 roll. So I feel the difficulty for secondary attribute should be a 14.
This is all just a first impression and needs some play testing. I'll follow up once I have tried it all out.