Mansfield 160 leaking around the 3 bolts that mount tank to

First determine exactly where it's leaking.

Wipe the underside dry with lots of paper towels including the bottom of the flush valve and supply line. Then wipe all the suspect areas with toilet paper. When your sure everything is dry, wad up some toilet paper and dab each area (start with bottom of flush valve) and inspect the toilet paper. Stop at the first sign of moisture on the paper and hold wad on that site. Take another wad and continue to dab other areas. Repeat until no moisture is found.

The reason you do this is that water will migrate and areas that are wet might not be where the leak is.

Dry all wet areas and hold paper on all but one that was wet. Is that one getting wet again? Yes---thats a leaker. No---not a leaker. Continue until you have eliminated all but one area that keeps getting wet.

I suspect it is really the flush valve or the supply line that goes to it. If this is correct then reinstall the flush valve making sure all rubber is positioned correctly and tightened uniformly (uniformity trumps tight in toilet repair) be sure supply line has the rubber grommet and is in good condition. (Not dry and cracked or pinched) supply lines are cheap and sometimes you can just replace the grommet.

If it's not the flush valve then you must have josteled the tank and probably damaged the old rubber seal kit. (hopefully you didn't crack the porclein) . Buy a new tank seal kit for your unit. Turn off the water, flush and hold lever down then bail the rest of the water out of the tank. Remove the supply line and remove the tank. Remove large rubber O ring from tank bottom clean and install new O ring. Clean bolt holes. There's a couple of different ways the bolt holes seal but they all require rubber pieces inside the tank and the bottom of the tank. Typically you slide a washer on the bolt, then a rubber seal. Slide bolt threw tank and put bottom seal on bolt, then washer, then a nut. To both bolt holes. Tighten uniformly until rubber seals slightly distort and seal. You can tape the large hole and put water in the tank and see if it's sealed at the bolt holes. Place tank gently in position and slide protective plastic rings under the toilet side and thread nuts find get tight till tank rests on it's own. Tighten them uniformly (one side 1/2 turn then other side and repeat until tank is level, straight and large O ring is seated. Reinstall supply line and test. *****Be very careful when tightening fasteners to porclein. Uniformity is key. Tightness is secondary to a good seal. You can tighten a little at a time after the fact and stop drips while everything is in place. BUT if you over tighten or don't use protective washers you might hear a minute crack. If so you have screwed the pooch and cracked the porclein from which there is no recovery.

If your uncomfortable with this procedure it might be well worth the $100 a plumber will charge then it's his porclein if you hear that minute cracking sound.

Uniformity of tension and not tightness of bolts seals toilets. Always finger tight every fastener and use 1/2 and 1/4 turns on one bolt then the other until the seals SLIGHTLY deform. If it drips simply use 1/8 and 1/4 turns while everything is assembled until each drip stops.

Good luck and finesse it.