Hi and welcome to TCS
I use a cattery regularly for my cats and it's definitely my preferred option when I go away for more than a few days. There are pros and cons of both catteries and cat sitters (either friends/relatives or professional sitter).
A sitter is good because cats are territorial and that way they can stay in their own home. You don't say whether your cats are indoor or indoor/outdoor but since you're in N Ireland I'm guessing they have some outdoor access? If you go away and leave them in the care of a sitter it's usually recommended that you keep them indoors for the duration as cats often go missing when someone other than their usual carer is in charge of them, and a sitter can't usually spend a long time trying to locate them and get them indoors to feed them etc. So their routine will be disrupted even if they are in their own home. My cats are indoor cats and if I had a sitter I'd worry about them getting out as I've only one front door. I'm used to them and know their habits but a stranger or friend would not. Also for me a sitter is a bad idea as Jaffa is very nervous and hides when strangers come around. So a sitter would struggle to get him fed (if food was just left out for him Mosi would eat it all) and would hardly see him at all so wouldn't pick up on him being unwell. Most sitters only visit once or twice a day for about half an hour so they are on their own a lot.
Advantages of a cattery to me are that I know they're safe (there are several doors between them and the outside so escape is very unlikely), I know there is someone keeping an eye on them throughout the day so that any problems can be picked up, and they get more attention than they'd get with a sitter. Mosi is young and lively and he got loads of play and fuss when he was in the cattery over xmas - much more than he'd have got from someone dropping by twice a day. The disadvantages of a cattery are that they're in a strange place rather than their own home which can be stressful. But Jaffa is a timid cat and although he's nervous and timid at first he does settle down and get used to the people and the routine.
I wouldn't worry about being gone for 3 weeks. Cats don't have the same perception of time as we do so they won't be aware that you're gone that long.
If you do go for a cattery make sure you check out several before you book and make sure they're clean, have sneeze barriers etc and that the staff are cat lovers and not just in it for the money. FAB have some guidelines on choosing a boarding cattery (just google FAB) and also have a list of FAB approved catteries, although I'm not sure whether that covers N Ireland or not.