I'm assuming your three inch pipe is two and seven eighths (two and a half inch pipe). If it's heavy wall then chances are it's what is referred to as "casing". If it's casing and it's new and you're gonna get it for a dollar twenty five a foot, do it.
Home Depot in Weatherford Texas is selling prefabbed corners and H's. I think they're wanting sixty three bucks for a corner. That's three pieces of heavy wall (three eighths) two and seven eights seven feet long for posts with a single four foot brace welded in between the posts. Of course they have the welded on caps too.
I wouldn't recommend them for squat, well maybe squat, if it was under nice pecan tree./w3tcompact/icons/laugh.gif
First thing is you need to be deep for a post to hold over time. The way I usually do a paying customer is place a pen between two of their fingers. I tell them to hold it tight. I move it. Then I have them grip it in their palm with their hand wrapped around it. I can't move it near as easy. It's an absolute law. The deeper the post the more secure it's going to be without it falling over due to moving in the soil.
So your ten foot posts are just about perfect. If you're not gonna use them for gates and your soil is good and stable then four feet in the ground and five feet out for field fence and three strands of smooth or barbwire.
The other problem with the corners and H's at HD in Weatherford is the cross pieces are four feet long. I like eight. It just seems to me that if you put the posts eight feet apart they are gonna be less likely to move under tension than if you have them four feet apart. I've also noticed tons of H's and corners failing when they're short. Of course I guess someone putting them in short would also put them in shallow, /w3tcompact/icons/smile.gif.
Chances are you haven't had much success coping the pipe to get a good saddle weld. Most folks make too much of doing that like it's hard or you might need some magic or something to do it right. Nothing to it. Of course I cheat. I've got mechanical notchers and dies that I use with my plasma or torch.
But I can do a nice saddle without the aids and you can too real easy like. The easiest way I can describe doing it is to think like you're sharpening both ends of the pipe at about kinda a forty five degree angle. You come back an inch and down an inch and low and behold there's an inch left in the middle when you're done to wrap around the pipe.
With small stuff it's easiest done with a grinder. But with the stuff you're gonna buy and be thankfull for you might had oughta use a torch. But the same principle applies as to if you were using a grinder or a saw. You're gonna make a perfect horizontal cut at a forty five from the top towards the end. Then you're gonna roll the pipe one hundred and eighty degrees and do it again. One end is done. You do the same exact thing at the other end making sure of course that you've got the pipe ends on the same plane.
Now if you make up the corners in the shop it's not a thing but putting them into the holes and leveling it all up and then putting in all the concrete you can beg borrow or steal into that hole that you dug was too big but might not be big enough after all.
If you set the posts and then want to cut the pipe to fit it's a snap. Measure the distance between the posts inside to inside. Then figure what you're gonna lose with your coping. With a half in wall two and seven eighths I'd guess three quarters of an inch on each end. So you add that three quarters of an inch each end to the inside measurement between your posts. Let's say it was ninety six inches inside to inside. You add your three quarters, two of them, and you have ninety seven and a half is the length you cut your horizontal piece.
I hope this helps.
And don't even consider driving in corners and H's. The fence gawd will turn your fence over to the fence devil and you will be soooooooo sorrryyyyyyyyy.
I'd also put in H's about every three hundred feet. Mostly cause they make most field fence materials in three hundred and thirty foot increments. They've been doing it so long so I guess they've figured out that works best. No sense in re invented the wheel, right?
