Mechs and CR ratings

[Wesrolter]

I am a little concerned about how my brain reads using Mechs.
The way I read it is you can use a creature 3 CRs above what you would usually use.
So For this example, when my players were level 7, I liked using CR 4 for a numbers encounter, Something that can hurt but isn't a huge threat. So using the same group, would I be using CR 7 for the same style of encounter?
The reason I ask is looking at Mechs available and their ACs are roughly around the 25 mark, a CR 4 ranges between a +10 to a +13 while a CR 7 is +14 to +16. Obviously, those numbers aren't accurate for everything, but they are a few examples of ABs for their CR.
The Mechs don't seem so well off defence wise against said enemies. So am I reading the CR section wrong?
my team is going to be using Mechs next session so I just want to ensure I don't over do enemies.

Edit to cover possible replies. I am after if the CR frame is correct, not if the defence and HP stats of Mechs being judged wrongly. I only use it to show why my brain has issues...

[Kamyr]

It's right that you up the CR by 3. It's even more of a 'guideline' than encounter CR for normal PCs, imo.

[Begin Longwinded Overexplanation]
The defense can go up a decent bit.
The max tier for L7 would be T8. Assuming 4 PCs, 420 MP to spend.

Base defense is 19 SP (8 regen), 4 hardness, 23 AC.
Legionnaire - 28 MP, 82 HP, 25 AC, 5 hardness
Biped, Heavy - 44 MP, 94 HP, 26 AC, 5 hardness
Tough Arms - 52 MP, 102 HP, 28 AC, 5 hardness
Let's say 4 at-level weapons with average 3xLevel for 96 MP, total 148 spent.

5 unbreakable armor - 212 MP spent, 102 HP, 28 eac/33 KAC, 5 hardness
5 rapid reflexes - 372 spent, 33 eac/33 kac, hardness 5
1 Enhanced shields - 396 spent, MP27 SP (8), 102 HP, 33 eac/33 kac, hardness 5
1 unbreakable armor for 115 HP

A CR7 combatant has to roll a 16, since the gargantuan mech is unlikely to have cover (though not impossible). 3 of those equals a CR 10 encounter. Without penetration, they're doing around 8-12 average damage with a 20% hit chance (50% chance each round they do no damage).

The minimum is solo T6 mechs with 23/24, 16 SP (regen 6 [+2d8 with replenish]), 102 hp, hardness 3 if they pick the lowest AC options and no upgrades to survivability. If the CR7s concentrate fire, they have a decent chance to do damage, but they'd have to get lucky and benefit from unforced errors to actually bring a mech down.

And then the pilot can abandon the mech and keep fighting.

[Metaphysician]

Yeah, don't forget that all mecha have both regenerating shields and hardness. That adds to survivability in a way separate from AC, and I'm pretty sure this is intentional- mecha are easier to hit, but harder to do real damage.

[JiCi]

They also kinda push you to build one mech for the entire party, or at least not giving each player its own mech.

You can build stronger mechs at a lower CR by having more than one operator. This is how you can compensate for lower HPs.