Sure, you can have three one ways on a ticket. A maximum of 8 flights can be had per ticket (or rather, two consecutive ticket #s, as each tkt # carries a max of 4 flights).

Either direction you do it it'll be the same (presuming same award flight availability both ways).

As Delta eliminated stopvers as of January 1, 2015, it will price as three one ways (or possibly more due to the introduction of married segment and resulting additive higher pricing; read this forum for more info on that).

Edit: this will likely price as four one ways (2 x MNL-NRT and 2 x SIN-NRT), as MNL-NRT-SIN is likely way over the MPM (maximum permitted mileage) for that route, so it won't price as a through trip with that illogical connection. If you want to pay for it as three vs. four one ways, fly MNL-SIN on GA via CGK, or VN via SGN or HAN, or CZ via CUN or, possibly, CI via TPE and, possibly, MU via PVG (but those are then pushing it for longer and may be over the MPM...and KE via ICN is almost surely too long of a route).