first of all, congrats to securing 3 offers - means you must be doing something very right! Here my thoughts to your questions..
1. Which offer prepares you the best?
Based on my experience, among these 3, the Accenture offer will give you the best odds / starting point to move up the ladder into MBB / Tier 2. While subject to regional nuances, Accenture has generally a stronger brand as a feeder and I know countless candidates who moved from there directly to MBB.
2. Can / should I accept first and reject afterwards?
I'd recommend you don't do so. Accepting an offer is a firm commitment and commitments should be honored - not doing so is often considered quite bad etiquette. Obviously, if you still chose to do so, not much will happen to you (beyond maybe burning bridges with resp. firms).
Now question is - will anything change from now until 3 months before you start? If it will still be choosing from these 3 options, then you may as well take a decision now. You know your ultimate objective, you know what gets you there easiest, and, having gone through recruiting, you may have a solid idea of where you may feel most comfortable (if not, speak to more people from these firms). These factors are unlikely going to change in the meantime.
Hope this helps - feel free to reach out, if you would like to discuss further.