On youtube...they were saying the Trident autothrust only applied to nos 1 and 3 engines...
Correct, but only for an autoland. For a manual landing, if autothrottle was used, it controlled all three engines.
During an autoland what happens to the throttle position of the no. 2 engine?
At around 1,500R, number 2 engine was disconnected from the autothrottle system and manually set at a typical approach RPM, where it remained for the remainder of the approach. For the Trident 3, this figure was 11,200 RPM.
The NHP closed number two throttle manually on main wheel touchdown.
Is this normal with all three engine jets and why?
The DC10 and the L1011 did not have this procedure.
As to
why the Trident did this, I don't have a definitive answer. Perhaps the designers were concerned about a high RoD building up near the ground if the autothrottle system failed or if it commanded too large a thrust reduction in gusty conditions or if the engines were slow to spool up. Remember the Speys were CTVN engines!
Constant Thrust Variable Noise 
The main reason we did it was.............because it said so in the AOM.
