UPDATE 2-Fortress, Lightyear, Crestview near Fla. bank deal

* Three firms to invest $450 mln in new capital

* KBW to raise $350 million from other investors

* First Southern seen as vehicle for bank acquisitions

* Small bank seen growing assets 12-fold to $5 bln (Adds details, background)

By Joseph A. Giannone

NEW YORK, May 28 (Reuters) - Three private equity firms are near a deal to inject $800 million into a small Florida bank, giving them control of a healthy lender that could be used as a vehicle for acquiring more banks, people familiar with the situation told Reuters Thursday.

Under a preliminary deal expected to be announced Thursday, Fortress Investment Group LLC (FIG.N: Quote, Profile, Research), Crestview Partners LP and Lightyear Capital would each hold a significant, minority stake in First Southern Bancorp Inc (FSOF.PK: Quote, Profile, Research), a Boca Raton-based community bank with less than $400 million of assets.

With the investment, First Southern could start gobbling up troubled banks in Florida. People familiar with the deal expect the bank could grow to at least $5 billion in assets.

The three investment firms would collectively hold a majority stake of $450 million, through individually the firms would remain below the 25 percent threshold that would then trigger Federal Reserve oversight.