The channel 3 has a function to slow down the servo, I believe to setup airplanes to behave like they have turbine lag.
There's a long faq on futabas web site that details making channel 3 the channel you want to slow down then mixing another channel to make it the throttle.
http://www.futaba-rc.com/faq/faq-9c-q762.html
Page 58 of the manual describes the throttle delay function.