The fire started around 11:45 p.m. Wednesday on the unit block of Fresconi Court.
Flames started outside one of the homes and then spread to the interior.
Officials say the fire then spread through the roof and extended to two neighboring homes.
Everyone was evacuated from the houses. No one was hurt.
The fire marshal says someone intentionally started the fire in one townhouse.
The damage is estimated at $300,000.