Upvote:2
When you have two passports you usually only present one to the country you're visiting. It's doubtful Australia cares about your other nationalities as long as the passport you present adheres to the rules about it.
Australia lets you apply for an evisitor visa online (subclass 651) if you hold a passport from Belgium, and for a visitor visa (also online, subclass 600) if you decide to travel with your Russian passport. Those are both different, you should apply for one or the other and once you decide which passport to go with, you should stick with that decision.
Also note that an ETA is not a visa and neither of your passports qualifies you for one.