The best fare available is through VIA's "multi-city" function. Select that here:
http://reservia.viarail.ca/tsi/tsiresul ... rom=iframe
Click "multi-city" and enter your three segments. The Supersaver or, in some cases, the Special fare is the best. I looked at some August dates at random and the best fare for the three segments is $142.38. There are cancellation penalties though, so this would be a good option only if your dates are firm. The best available fare with no restrictions - Regular fare - is $207.92. There is no advantage to booking that too far in advance, other than to ensure you get on the train of your choice should it be a busy travel period. If you wish to splurge on Business Class, the multi-city function works for that as well.