Can I set different objectives for ad campaign that boosts registration versus subscription versus product purchases from my web site?
I think we need more detail on your query as objectives in AMO are assigned at portfolio level and Yes portfolios with different objective can be created and campaigns with same objectives can be assigned to those portfolios. So in your case above three portfolios can be created and budget can be specified accordingly. It will help in managing the campaigns easily as the objective will be same so technology will work in optimizing campaigns towards that and will also ensure the budget is spend as assigned and per objective.
Hope this helps?