Take a look at VTI (Vanguard Total Market ETF). If you don't understand ETF's, do a quick internet search on them. If you want to have your trade executed mid-day, this is what you need to buy. It is basically the same thing.
ETF's trade like stocks - if you have $3,000 to spend, you have to figure out how many shares you can buy at the current bid price, then put in an order to buy the shares.
It may be helpful to search "VTI vs. VTSAX".