You can take a bus from Tres Cruces to Colonia and then a ferry to Buenos Aires. You can book both ahead of time. I don't think bus prices are cheaper if you prebook, but ferries could be cheaper as the lowest fares sell out first. Be careful if booking far ahead as the highest fares are released before the lower ones.
I just got back from a trip to BA and Montevideo. What worked well for me was a one-way ticket on the Buquebus ferry from BA to Montevideo for a few days, then a bus (about 2.5 hours) from Montevideo to Colonia (there are lots of buses from Tres Cruces station in Montevideo every day), then a one-way Buquebus ferry back to BA. It's easy to book a Buquebus ticket online even with poor Spanish, and they often sell out.
I stayed overnight in Colonia, but if you were to take an early bus from Montevideo and catch the 4:00 ferry back to BA, you'd have enough time to see Colonia.Edited: 6:50 pm, November 18, 2017
Thanks eh! Fellow Canuck here living in the US. Thanks for the thorough response.