I think it is a good idea to focus on how wide their opening range is. I believe this is one of the most important factors for deciding to 3-bet. Don't just 3-bet, because everyone else is doing it. Versus many nits it is best just to 3-bet KK+ and AK. Even sometimes flatting AK might be better. Versus much looser opens you should be 3-betting 99+, AJs+, AQo. If they have the ability to fold to future aggression you can add some suited hands that can't call as bluffs. Don't get carried away with this. In your post you said people aren't folding. So stop bluffing
them. That means don't add J6s or 62s as bluffing hands.
Also, use position to determine their range. When you are dealing with readless opponents I 3-bet a fairly narrow range since most opponents will call it.
When 3-betting from out of position I don't think it is horrible to only do so with value hands.
If your getting different reactions from people I don't think changing your bet sizing is the right approach. I think focusing on changing the type of hands you 3-bet.
When out of position I like a 3x 3-bet rather than a 4x 3-bet. 4x allows them to fold more hands without feeling too bad about it.