So you have three HDDs now ? Each having a sing copy of OS installed ?
If so, it is the best practice to use the the BIOS Boot Menu (F8 for Asus motherboards). After power on, when the motherboard splash screen appears, tap and hold F8, it will list up all the possible drives to boot from. Select which one you want by the scroll arrow keys, and enter to boot from it. Let the MBR not to be messed up.
But if you want to use a third party boot manager, you may opt for the free version of EasyBCD. Get it from
EasyBCD - Take control of your boot loader | NeoSmart Technologies .
To add all the three boot entries to MBR , run
Startup Repair with all the three HDDs plugged in.