Almost six months after the introduction of the iPhone 4S, the flagship device is continuing to be a strong seller for AT&T, Verizon, and Sprint.

According to AllThingsD, the iPhone 4S is believed to be the top seller on all three carriers for the month of March. And for Sprint and AT&T, the iPhone is really out in front of its Android competition:

“Our March checks indicated the iPhone continues to extend its market share gains,” Canaccord Genuity analyst Mike Walkley writes in a note to clients today. “In fact, we believe iPhones are outselling all other smartphones combined at Sprint and AT&T and selling at roughly equal volume to all Android smartphones at Verizon.”

The Motorola Razr/Razr Maxx and Galaxy Nexus were the second and third most purchased devices on the Verizon network in March. The Samsung Galaxy S II and Galaxy Note trailed the iPhone 4S on AT&T, while Samsung’s second generation Galaxy S phone was also the second best seller at Sprint for the month.

Surprisingly, the 8GB iPhone 4 was the third-most purchased handset at Sprint in March.

Walkley also said that Samsung should continue to cement its second-place standing in the market with strong growth in the most recent quarter:

“Our global checks also indicated another quarter of strong market share gains for Samsung, particularly in Europe and Asia,” says Walkley. “In fact, we believe Samsung’s strong smartphone portfolio across all price tiers will result in Samsung’s smartphone shipments increasing a remarkable 15 percent-plus sequentially during the seasonally weak March quarter.”

Did you purchase an iPhone in March? Hopefully you didn’t take our April Fools’ prank too seriously and start this month by purchasing a new Android handset.