Company D was the weapons company of the regiment's 1st Battalion (each battalion consisted of a headquarters, three rifle companies and a weapons company). This means, unless he had a support MOS (military occupational specialty), he was probably assigned to a crew-served weapon (machinegun, mortar, etc.).
Unable to determine whether the regiment's veterans association is still active but you can contact Third Marine Division Association. They're at and you can e-mail their webmaster, Bill Ervin, at ; he can inform as to how you can post a request for info, etc., at their website.
Muy importante! Third Division may not have been the only division he served with. The marine corps fielded six divisions by the end of the war but no more than two or three were ever at full strength at any one time (this usually just prior to a combat landing).
Individuals and even entire units (usually company-strength or smaller) were on several occasions transferred in to bring a particular division up to full strength in preparation for an upcoming operation. An example: in February 1944, all marine raider battalions were disbanded, primarily to free up personnel to form the Fifth Marine Division.
I once researched a man who landed at Guadalcanal with First Marine Division and transferred to Second Division (at his own request) when First Division was relieved.
He landed at Bougainville as an attached observer with Third Division and at Betio (Tarawa) with Second Division. He somehow wound up with 22d Marines - then principle element of 1st Provisional Marine Brigade - with which he landed at Eniwetok.
He remained with the regiment when it was used to form Sixth Marine Division and landed at Okinawa with same. His service, although unusual, wasn't all that extraordinary.
Small, seemingly insignificant items like many of the above are nice to know and always helpful - sometimes even crucial - when locating info.