Clarke, Spencer and Powell will be training with the squad over the weekend as the Bees prepare for Monday's clash at Dagenham.
Forward Spencer signed on loan from Cheltenham until the end of the season on Friday to give Scott's men a better chance of increasing their lead at the top of League Two.
Fellow striker Clarke joins on loan from Ipswich until the end of the season to become the third Town player to move to Griffin Park this term following Dean Bowditch and Jordan Rhodes.
And the striker will provide a huge boost to Scott's attacking options after Rhodes was forced to join Nathan Elder on the sidelines when he suffered a metatarsal injury in Tuesday's 1-0 defeat to Chesterfield.
Meanwhile, central defender Powell, who spent four years with Brentford between 1998 and 2002, has signed a two-month contract with the club following the end of his trial at Leeds United.