once they have three out, however, they unlock a series of "rune bursts" - the abilities basically use up the runes to deal big single target DPS, AoE DPS, or enemy debuffs, depending on the rune burst ability and/or the runes that were cast. Again, no casting time since you took time to build the runes in the first place. Once the rune burst was activated, the existing party buffs from those runes remain for another 15 seconds and then dissipate, which is when you can recast a new set of three runes.

They'd also of course have some fairly low damage instant cast DPS or other emergency skills to use inbetween rune building. Possibly some longer cooldown ability that eliminates the cooldown between runes, if you need to get a specific rune burst out. possibly after 50 they can build up to four runes that unlock a series of new rune bursts?