Welcome to the board, CS!
By asking this question so early in your career, and at your age, you're about a decade ahead of most of this board's servicemembers & veterans. You'll have enough compounding time on your side to really make a difference, whether you choose the TSP or a Roth IRA.
I am interested in starting a Roth IRA, I just need some advice. Should I continue contributing to my TSP until I exit active duty (3 years from now) and then convert it to a Roth? Or should I start the Roth now and stop contributing to the TSP? Or should I try and contribute to both now?
Conventional wisdom is to max out the TSP because it has lower expense ratios than any other index fund-- at 0.03%, it's even lower than Vanguard.
https://www.tsp.gov/investmentfunds/fundsoverview/expenseRatio.shtml
Over the decades those tiny little basis points can add up to a substantial difference.
Some servicemembers are reluctant to invest in the TSP out of a concern that they "can't get" the money when they want it. As you've mentioned, however, you can always roll out of the TSP to a conventional IRA and then start a 72(t) withdrawal, even before age 59.5. What you'll probably want to do, however, is keep your TSP account after you leave active duty and just let it compound.
Beyond the decades of low expenses, keeping your TSP makes it a lot easier to resume contributions if you decide to re-enlist or to join the Reserves. In addition, the TSP is offering a Roth TSP option. The legislation was enacted a couple years ago and the announcement came out last week:
https://www.tsp.gov/PDF/formspubs/oc06-5.pdf (Implementation may lag into 2012, however.) This means that you could put your Roth IRA contributions into the world's largest index mutual funds at the same 0.03% expense ratio.
When this feature is in effect I'm going to roll my spouse's entire Roth IRA over to her TSP account. Now I'm really regretting that I was kicked out of the TSP when I retired from active duty.
Here are some other points to consider on where you invest your money:
Where to put your savings while you’re in the military | Military Retirement & Financial Independence
Tailor your investments to your military pay and your pension | Military Retirement & Financial Independence
Start saving early | Military Retirement & Financial Independence