All 3 channels are on the same transponder on Sky - are any other channels affected?
What make and model of box? - "Services" "System setup" "System details" - first and third lines please.
Possibilities if only one transponder
1 Interference from some Wi-Fi device or DECT phone
2 Failing lnb
3 Faulty cabling - with kinks, joints or wallplates
4 Failing dodgibox
Try
1 "power reset" - switch off at mains for a minute or so.
2 Try also a "forced software download" - Swtch off at mains, press and hold "backuo" button on box front, switch back on at mains and hold "backup" button until either you see an on-screen message (if connected by scart to TV) or four LEDs light up on box front, go and make a cuppa as it takes 10 to 15 minutes, when download is complete the red standby led will light up, wait 30 seconds and turn back on with remote.