The easiest way would be to use a side grinder and cut the head off the bolt. Then you can take it out with your fingers. (after it cools)
The blade bolts need 120 #ft of torque. Use 80 #ft and it may take 280 to get them off. Use 120 and it will take 120 to get them off. Too loose and they will work tighten.
Considering the age of your mower you should replace the other 2 bolts.