Obviously I used three different transponders for the three simultaneous records, so I expected to be able to change channel in the channel list only for those of the three transponders already tuned, but I saw all channels available.
Anyway, I've tried another time after a reboot and this time the channel list is right, allowing me to change channel only between channels of the same transponders used by the three tuners.
Maybe this bug appears only under certains conditions...