Hello Llijaullo51
This train is not fully booked, currently about 90% of the seats are free. I think that reservations are not possible between Domodossola and Basel for this train, but I don’t except that a lot of people will travel with this train, or perhaps from later stops, but you will already have your seats. There might also be people leaving the train at Brig and freeing up seats. It is very unlikely that you have to stand for 3 hours.
Kind regards
Danhiel