Minimal stimulus, sitting on my duff & stuffing my face is what worked for me, are you eating BIG?

What did you 3x per week training program entail?
How much are you eating?

FWIW - I hike, bike, splitboard and hunt. I'm 6'1.75", 230lbs.
I don't run.

Getting big and running long distance are contrary goals.
If you want to be a good fighter you don't want to run long distance. You'll hit like a kitten. 

I'm not an ectomorph. I'm an endurance athlete that carries a lot of muscle.
I was a super endo-mesomorph as a kid. I've been through a few different physique stages over the years.
If you aren't doing big milage on a consistent basis, right, it's not a big deal.

On paleo I can eat around 4-5 thousand calories per day (bw 180Ibs) while training, not sure if/how effective bulking on paleo will be . 

Paleo, with a higher calorie intake, should be very effective in adding muscle mass. Highly nutritious and faster recovery times!

Personally speaking, I have an ectomorphic body type and have gained strength and some mass in the process. However, I've been training for gymnastics strength so how much potential muscle mass I could gain through a free weights strength/hypertrophy routine, I can't honestly say.

I'm 6'8" and 200lbs;  tall and skinny.  I'd like to bulk up a lot and increase my strength particularly for short duration activity (fighting).  Trouble is, I've been working out for years and the best I ever did was to get up to 218 lbs.  My theory is that I've been doing too much exercise.  I only go to the gym for an hour or so at a time but I enjoy things like hiking and canoeing and cycling.  Do I have to cut out time at the gym if I go for a hike?  What if I'm out for a few days in the mountains?  Does being a muscular ectomorph preclude doing prolonged exercise?  I'd hate to have to choose.

In case you are wondering I have been on paleo for 2 or 3 months.  When I was 218 all I did was lift weights 3X a week and did one 5-7 hour run on weekends FWIW.  Rest of the time I was sitting on my duff.

You sound like a kindred spirit. I've always been in pretty much the same boat, though not as tall as you.

I'm 6'2" and hovered between 155 and 165 for most of my adult life. The turning point for me was when I discovered High Intensity/Abbreviated training.

Learning how to squat and deadlift, adding weight on the bar every week, excluding virtually all isolation exercises and cutting my gym visits down to once per week were what finally did the trick.

Right now I'm about 200lbs and fairly lean. I was actually up at 235 about a year ago but drinking like a fish, eating like a pig and getting fat.

My currently program has me lifting progressively only once a week, though I also do a few chin-ups and pushups just to keep the blood pumping between heavy sessions (active recovery). I also do Kickboxing/JKD twice per week and SAMBO/Submission Grappling once-twice per week. My "cardio" comes mostly from walking and Kickboxing recently, as I hurt my foot barefoot jogging a couple of months ago.

I don't think you necessarily need to cut out your fighting, hiking and canoeing but the only way to know for sure is to try it. If you can still add weight to the bar every week then you're moving in the right direction. If you're not lifting any heavier, then you're not getting stronger and you're not gaining any muscle. That's the bottom line.

The impotant books for you to check out would be...

Beyond Brawn by Stuart McRobert
Power Factor Training by Pete Sisco and John Little
and
Dinosaur Training by Brooks Kubic

Mike Mentzer's stuff is also pretty good for this kind of approach.