Is a fitness training routine combining rowing, treadmill cycling good?

Yes: You are nicely combining three different forms of aerobic conditioning exercises. Rowing and cycling activities are heart friendly as well as joint friendly; good exercise for the cardiovascular system without beating up your joints. Of the three, the treadmill has more impact on your knees and ankles as you run. Would advise keep in the treadmill flat as opposed to inclined. Easier on the knees.

Yes: Sounds good as long as you mix them up in different sequence every so often. Would help to add some weight /resistance training in there as well. You are getting some with the rowing. Stay on it and enjoy it. Good Luck. Also good to get outside and do some.