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Ogres receive 7.5 HP per HD (d8+3).
You want to increase him from CR3 to CR6, that's 40 HP, so 40/7.5 = 5.3 extra HD, let's round down to 5 extra HD. So he goes from 4 HD to 9 HD.
At 9 HD, he would have 67 HP, that's 37 more than the standard Ogre.
Good so far?
It also says that creatures whose HD increase by 50% should also increase in size, so your ogre should become Huge. That adds +8 STR, –2 DEX, +4 CON, +3 Natural AC.
That is going to add 18 more HP which takes it up too high, so now I suggest knocking down the HD increase since we don't need all 40 HP to come from HD (some comes from new CON bonus). Let's try 4 HD instead of 5.
His new HP at 8 HD is 78 HP, that's 48 more than the base ogre. We only wanted 40, but if we lose one more HD, his HP goes down to 68 which is lower than what want, so let's stick with 8 HD. He's likely a solo boss; he needs the help.
With 4 new HD, he gets +1 to an ability score. I suggest STR, bumping it to 30 (21 + 1 + 8 size increase). Now he can hit very hard.
Still following?
For skills, he gets just one skill rank per HD, so let's split them on the skills he already has, +2 Climb, +2 PER.
He gained 4 HD so that's two feats. You could pick anything, but I borrow inspiration from a Hill Giant and kept it simple with Power Attack and Cleave.
His new BAB jumps up to +6, CMB is now +18, CMD is now 26 (adjusted for size increase and new STR).
I did the rest of the math.
Looking at the first chart on the Monster Advancement rules, we see the last three columns tell us the average amounts he should have increased. His AC only went up by +1 (+3 Nat, -1 DEX, -1 size) but the chart says he should get 4 points of AC. His attack bonus should have gone up 5 but it went up by 9 (3 BAB, 6 STR). His average damage should have gone up by 10, but it went up 12 (more if he hits twice). So it seems we didn't get enough AC but we gained too much attack and damage.
We could leave it like that, easy to hit, easy to kill, but blast also very possible to kill a level 6 PC (2 hits for average 57 damage, or 69 if he power attacks). Very dangerous. I don't like him being quite that dangerous (quite a bit more damage than a Hill Giant and the Hill Giant is higher CR).
But that is exactly what the rules say to do.
Here is his final Stat Block:
XP 2,400
CE Huge humanoid (giant)
Init –2; Senses darkvision 60 ft., low-light vision; Perception +7
DEFENSE
AC 18, touch 6, flat-footed 18 (+4 armor, –2 Dex, +8 natural, –2 size)
hp 78 (8d8+32)
Fort +9, Ref +0, Will +4
OFFENSE
Speed 30 ft. (40 ft. base)
Melee greatclub +16/+11 (3d8+15) Power Attack: +14/+9 (3d8+21)
Ranged javelin +4 (2d6+10)
Space 15 ft.; Reach 15 ft.
STATISTICS
Str 30, Dex 6, Con 19, Int 6, Wis 10, Cha 7
Base Atk +6; CMB +18; CMD 26
Feats Iron Will, Toughness, Power Attack, Cleave
Skills Climb +15, Perception +7
Languages Giant
But as I said, I don't like him being that deadly, or that easy to kill. So I would make a couple changes.
1. Swap 2 points of STR to CON. This makes him a little more durable and reduces some damage. He's still stronger than a hill giant.
2. Take that +1 ability score I put in STR and move that to CON. Even more durability.
2. Add 3 points of additional natural AC. Just because.
This changes him to the following:
XP 2,400
CE Huge humanoid (giant)
Init –2; Senses darkvision 60 ft., low-light vision; Perception +7
DEFENSE
AC 21, touch 6, flat-footed 21 (+4 armor, –2 Dex, +11 natural, –2 size)
hp 94 (8d8+48)
Fort +11, Ref +0, Will +4
OFFENSE
Speed 30 ft. (40 ft. base)
Melee greatclub +14/+9 (3d8+12) Power Attack: +12/+7 (3d8+18)
Ranged javelin +4 (2d6+8)
Space 15 ft.; Reach 15 ft.
STATISTICS
Str 27, Dex 6, Con 22, Int 6, Wis 10, Cha 7
Base Atk +6; CMB +16; CMD 24
Feats Iron Will, Toughness, Power Attack, Cleave
Skills Climb +13, Perception +7
Languages Giant