In some of my parts I’m moving a lot of my layup is 3 ply of 3k carbon. If I were to switch to 1 ply 3k and 1ply 6k will My parts have close to the same ridgity, strength and weight as 3 ply of 3k?
Depends on the weight of 3k and 6k you are using. If the 6k is double the GSM of the 3k, then yes, it will be very similar assuming the weave type is also the same. For woven twill fabrics, the thickness is similar to the weight. a 200gsm 3k will be ~0.2mm thick, a 400gsm 6k will be ~0.4mm a 660gsm 12k will be ~0.66mm… Of course this also depends on the process and actual FVF, but assuming standard vacuum infusion or prepreg process you can roughly go by this rule.