If I'm reading this correctly, this patch is the Radio Platoon of Communications Company, Headquarters Battalion, 3rd Marine Division.
The Third Marine Division Association
History of Headquarters Battalion
.....In March 1956 the Battalion moved to Okinawa and remained there in a readiness posture until 1965.
On 6 May 1965, the 3d Marine Division opened the Marine Compound at the Danang Air Base, Vietnam. The Battalion operated in Vietnam from this time participating in operations from Danang to Phi Bai to Quang Tri/Dong Ha Combat Base. For Meritorious Service in the Republic of Vietnam during the period 1 January – 30 September 1968 the Battalion was awarded the Meritorious Unit Commendation. The Battalion departed Vietnam in November 1969 and moved to Camp Courtney, Okinawa, where it is presently located.
Not sure if this patch is before mid 1965 or during their Vietnam service '65 to '69 or after they moved to Okinawa in November 1969. Anyone familiar with this one?