Thanks guys, job done, took 2 days but i got all three of them out. Going to replace the bolts with SS studs and nuts. Thanks. 

I just came across new product . kind of super PB rust buster.. $18 a can  from germany.. ( NOT legal for sale in the USA..)been using on Triumph TR 250 that I am sending back to the UK.. works great.. will post photo of it.. later bc

if you have access to an oxy acetylene torch.

Heat the studs till cherry red and let them cool without quenching. the studs will shrink when they cool. then you can usually add some P B  Blaster and let it soak overnite if they don't come out after the heat shrinking.

BTW,  the best way to remove a bolt or stud it to tighten it a cinch, then loosen it.

And if you've not already done this, double nut the top of the stud. Tighten the nuts against each other, and when removing, put a wrench
on the lower nut.

Careful with that torch if the engine is still in the car.

Thanks everyone, Ill try them all.

Often times it is the bolts that have seized to the thermostat housing due to galvanic corrosion-- the gap between the steel bolts and the aluminum housing being filed with white corrosion.

I've sometimes had to resort to cutting the heads off the bolts and trying to get the housing to hammer upwards. If that proves unsuccessful, I've split the aluminum housing off from around the bolts until I can get the housing off, then I use a small pipe wrench to grab ahold of the bolts-turned-studs and spin them out. I always replace with studs rather than bolts.

good luck

Heat the sides of the head where the bolts are with a blow torch, don't worry about saving the thermostat housing they are delicate at best and cheap as chips.

WD40 will not help any. You would want to use a penetrating oil, but there is no good way to get any where it will help. Try to tighten it a tiny bit of movement, then loosen. 

Head to Lowes, Home Depot, or your local auto parts store and buy a can of PB Blaster, Liquid Wrench, or similar to use instead of the WD-40.  WD is not as good a penetrant.  If you can find it, Kroil works really well.

Once you spray the T-stat housing and its bolts with penetrating oil, strike the heads of the studs/bolts with a hammer like you were driving a nail.  Give it overnight to soak in.  Before you attempt to loosen, strike the studs/bolts again.  Then try "tightening" the nut/bolts a tiny bit first, then turn to loosen.  Repeat the wiggling between tighten and loosen.  Once they start to loosen, flood with more penetrating oil, getting as much in the gaps as possible.  For every turn you are able to loosen, tighten it back 1/2 turn after soaking with more oil.  This repeated "in/out" turning will work the oil down into the threads and help prevent seizing as you remove the fasteners.

Any real nice way to get the bolts loose without breaking them? Got the eng. hot, WD40 over night. Not sure what the breaking point will be. Any help will be much appreciated. Thanks.