Circlet question

Expanding on that just a little bit, the 2 socket limit only applies to Circlets and Coronets. Diadems and Tiaras can get 3, either as a normal socketed item or an Artisan's/Jeweler's magic item.

Chromatic would give you the desired ideal 21-30% all res.

Since Artisan's/Jeweler's and Chromatic are both prefixes however, they can't be on the same magic item as Khegan already noted.

Generally speaking, you can often "downgrade" and go for a rare. Prismatic for example also gives all res, albeit only 16-20, and can be found on rare items as well and not just magic. Since rares do allow for more than 1 prefix on the same item, that approach would possibly work. The problem in this specific case being that the only socket prefix that can roll on a rare is Mechanic's which only adds up to 2 sockets.

Your possible alternatives, depending on what exactly you wanted to add to the sockets are as follows:
1) Go for a rare with 2 sockets, up to 20 all res and other stats that might make up for the missing 3rd socket.

2) Go for a magic Chromatic to get the 30 all res.

Larzuk it (and hope he is kind enough to give you 2 sockets and not just 1 - random roll there). That way, you have 30 res and 2 sockets at least. And the item can still have a suffix rolled as well which may be enough to make up for the third socket.