Don't know if I mentioned this already, but when you get a blade nut removed like on your 3 blade deck, wire wheel the threads clean, and put some copper based anti-seize on the threads to keep them from rusting, and to allow them to be removed without the 4' cheater bar. With that long a bar, if something won't give it's going to break, and in the case of a mower blade bolt that would make for a much bigger problem.
If I were you I'd get a big enough compressor to drive an impact wrench, which would also allow you to get other impact tools, like air driven cut off wheel, air chisel, air drill, etc., all of which are very useful tools to have for a DIYer or shady tree mechanic. HA!:laughing: Get it?, shady tree mechanic, HA!:confused2:
Seriously, a good compressor also allows you to spray paint. Battery or corded impact tools, not so versatile as what the compressor would provide for a long time.
My probable approach to your tight blade bolts would likely be: spray with penetrating oil, smack the head of the bolt with a metal faced dead blow hammer then use an impact wrench or impact socket with breaker bar and possibly a shorter cheater, 1 or 2' long. No success use heat wrench and smack it again, the same, air or breaker/cheater, until it gives. Then clean threads, anti-seize and tighten to torque spec for the bolt or until real snug + 1/4-1/2 turn. Note, anything applied to the bolt threads other than dry threads affects torque outcome. So with anti-seize on the threads you would want to decrease the torque enough to not over tighten the bolts.