Is it possible to serve in multiple branches of the military ?

Former Marine, at my first ODA in 7th SFG there were 5 of us former jarheads to include the team leader. Before I joint the Army after leaving the Corp, I visited the AF recruiting office...I was turn away for been a former jarhead. It all worked out in the end. Met many guys with two or more prior service in different branches, so its doable not really practical if you want to advance.

Not simultaneously, but when I was in A-school we had a transfer from the Marines who I want to say was a Staff Sargeant.

Yes I served in USMC,USCG,ARMY NATIONAL GUARD for Alaska and Washington state and did a combat tour in iraq 2004 - 2004. Going into the Marines first eliminating going into any other branches boot camp

ME US ARMY 1978 TO 1982 AND 1982 TO 1992

My son was in Marines for 7 years and transferred to Army last year. He has a lot more opportunities with Army than was available in Marines.

I would say not at the same time; however, joint service may be as close as you get to serving in all of the branches.

I heard back in the late 90's you can go from Air Force to Army but not the other way around....I think after being in the Army that you can't serve any other branch...Just what I heard talking to recruiters

I served in the Marines 92-96 as a 4421 legal pogue. After EAS I activated myself on IRR and lateral moved to 5811 MP. Did a year and change them left the Marines for good.

2005 I signed up for the Navy Seabees as a Steelworker and kept E4 and advanced to E5 before deploying to Kuwait/Iraq in 07.

As a truck driver you'd easily slide to seabees and youd also pick up experience in other construction equipment. Dump dozens scrapers graders excavators etc etc.

It's good to plan but don't neglect the present. When you are a year from EAS then start planning your move.
FYI
I had a blast in the seabees.

Yes Navy 1964 to 1968. Army reserve 1969 to 1993.

Anything is possible if you desire it enough! I originally started in the US Naval Sea Cadets when I turned 14, and then enlisted during Viet Nam as a Data Processing Tech in the Navy. I made it to E-5 and finally switched to the Naval Reserves after the war. I made E-6 and went up a number of times for E-7. However the rate had closed with virtually no promotion potential. I looked for other career options and was accepted into a "Special Warfare" unit on the condition that I would cross-train and swich rates to "Gunners Mate". I worked in the armory for several years, completed the requirements for conversion and made GMG1 (E-6). I was assigned to a PBR "boat" crew and eventually was "Riverine" qualified. When the "Riverine" community was merged into "Naval Special Warfare" I was qualified as a "Combatant Craft Crewmenber". Finally, I started having some health concerns and was told to consider a desk job. Being a member in a "closed loop" Special Warfare Unit, I was allowed to approach other branches of the service to see what they might offer. I was offered a "Ranger Slot" in the Army National Guard in a Battalion that was reclassified as "Air Assault". I accepted the transfer as a Staff Seargent (E-6), was given an ASI="G", and then in my late 40's, successfuly completed "Infantry School". I was assigned to the Recon Platoon as a Squad Leader and after 5 years in the ARNG I finally took retirement. My total span with the military was 36 years. Have Fun!