I gotta question.
WHY are you putting motors in the outboard tubes?
if the answer is, ”I want more altitude” then by all means go with the Ds.
if the answer is, “I want it for realism or just because it looks cool”, I’d go with an 18mm 1/2A or even a 13mm A something-T.
24, 18, and 13mm motors will ALL give you a flame and a smoke trail at lift off and for most of the altitude the rocket is in visual range. if you DON’T want or need the extra OOOOMPH for altitude or payload, the smaller motors give you the visual effects
WITHOUT
as much excess altitude you may not want or need
as much motor cost
As much excess weight in the tail which will require bigger fins or nose weight or both
risk of cattywampus flight if one motor doesn’t light (actually, if the main doesn’t light and your D boosters DO light, you have fecal turbine interaction because that puppy is leaving the rod and unless you ducted the outboards to the main tube, you have no chute deployment and it is coming back ballistic . This is considered poor form [even worse than ejected cases!]. OTOH, if you go with 13mm outboards, particularly a 1/2A or 1/4A, probably won’t even get all the way up the rod. I will take that embarrassment over a ballistic return any day.)
as big size vent holes. I don’t know what size vents are needed for a D motor, but given the outboard motor tube length is short (so not a lot of room for compression) I’d say they need to be pretty big. Are the outboard tubes 24 mm? If they are LARGER, than put your 24 mm mount in with longitudinal balsa strips instead of rings and you can vent out the back. Think Lil Augie spacers
http://www.spacemodeling.org/jimz/eirp/eirp_10.pdf
this is IMO definitely the best way to go if you use 18mm or 13mm motors, run the mount BT-20 or BT-5 almost but not quite to the cone (leave a cm or two). Gases go forward up the motor mount then back down between the mount and the outer tube. No visible vents, and acts as a baffle so if you have plastic tail cones on the outboards, gases should be sufficiently cooled.
Put some JB Weld or other high temp substance on the base of the outboard cones (which are gonna get blasted by the ejection charges, less so but still substantial burn through if you use zero delay) to protect them from the blast.
Another option is to put shock cords on the nose cones, use the same delay on the outboards as the main, example C6-5 and two D12-5s. Since the propellant burn time is similar for Estes 18mm C and D
https://www.rocketreviews.com/compare-estes-c6-to-estes-d12.html
the motors will all fire at approximately the same time. Use long kevlar shock cords with short pieces of elastic for the outboards, and an anti zipper device on all three shock cords.
@BlaineS has a great one
https://www.rocketryforum.com/threads/blaines-kevlar-anti-zipper-using-foam-ear-plug.154996/
so what if they don’t go off simultaneously, the rocket is gonna be pretty high up regardless, one or two secoconds shouldn’t matter. This solves your vent problem.