If I had a crystal ball and could give you an accurate answer to your question, I would be off on my own personal island somewhere in the Pacific and not be answering your question. That's a long drawn out way to say that in the world of investing there are not a lot of guarantees or fast returns. With that in mind, here are a few things that are important to consider as you review your options:
The TSP is long term. This employer retirement account offers tax advantages that encourage us to stick with it for the long haul. Generally, pulling money out (in certain situations where you can) before retirement (again, generally age 59 1/2) can result in taxes and penalties. Ideally, whether you're serving or are separating you'll be able to fence this money off for it's ultimate purpose: retirement.
Three years is a short time. When it comes to stocks, bonds and other investments, three years is not a lot of time. The reality (which has become readily apparent in recent weeks) is that market-based investments can go up and down and are probably not appropriate if you've got such a small amount of time before you need the money. That's why whenever I get the chance, I emphasize the link that should exist between the why (your goals) and the where (how you invest for those goals) when it comes to your investing.
"L" funds offer professionally developed portfolio. These Lifecycle funds offer a combination of the available TSP Funds (C, S, I, F, and G) based on your anticipated retirement date (not just from the military, but your final retirement) you select. The longer off that year is, the more aggressive (more stocks) the portfolio will contain. This can be a convenient way to build a portfolio, however, as I said earlier, there are no guarantees with this approach.
In the end, I can't provide an answer to your "fast return" question. I can say that making regular contributions to the Roth TSP is a great way to build for your future. Remember, you can always give on of our advisors a call at 800-531-8521 to discuss your specific situation.