your hardiness zone is the bottom number range of where most plants will do ok and your heat zone is the top number range where they will be ok concerning heat.
a plant that says it is hardy to zone 6 likely won't grow in zone 3..so you'll be throwing away your money..trust me...I live in a zone 4/5 combination and if I plant a zone 5 plant I am giving it a wing and a prayer putting it in..generally they do OK..but I have lost a lot of plants..
some hardiness maps have us in zone 5, some 5 b and some 4 and most winters if we have a normal winter with good wet snow cover everything is alright..but when you get a cold windy winter with thin snow cover..you can lose plants faster than scat !! so disappointing..there was a 10' white pine that didn't make it over this winter..probably wind burn when it was -30..
some plants can be grown in a
greenhouse in a lower zone, and some can be grown in a microclimate in a lower zone, like up against a south facing wall with some protection over the winter of maybe some
hay sandwiched between some
chicken wire..but you are taking a chance even then with 3 zones difference