First, forget the assessment. Assessments don't matter.
You're right that it's not easy coming up with good comps. I found the house you're considering and looked at comps within about a 1.1 mile radius--out to Viers Mill Road and down to the Beltway. And there were a few on larger lots. The difficulty is that many of the other recent sales are somewhat more modern ramblers or substantially newer colonials.
I'm not licensed in Maryland and while I know Kensington, I'm no expert there. But what I saw on MRIS tended to be in line with what those three Kensington Realtors told you.
In addition something that's only a 3-minute walk to a commuter line raises its value substantially. Properties within, say, a quarter mile of the VRE (in Virginia, where I can speak with some confidence) are worth distinctly more than the same properties a mile or two farther away. And that's the same with being close to Metro. Or a quick drive to get on the Beltway.
The extra land counts for something, but not a huge amount. For example, a house on 0.75 acres of land wouldn't be worth 50% more than a house on 0.50 acres of land. And even if you were to consider the assessment (which you shouldn't), in your case the land and the improvements (the house) are valued about equally.
If you don't have a Realtor representing you (I'm not sure about the standing of those 3 you spoke to), you should.
But, as I said: assessments don't matter. Location does matter. Amount of land matters somewhat, though really more to you (because of your dogs) than to someone who'd be just as happy on a third of an acre.
Recognize, too, that if you're having a challenge coming up with a "fair" price, the seller and his agent probably went through a similar exercise. So it's possible their number isn't as "solid" as if they were selling a cookie-cutter home in a large community. That means there may be more negotiating room for you.
So select a Realtor, if you haven't already, and have him/her do the best he/she can on comps. Then go from there.
Hope that helps.