ok, so it's an msi m/b

https://www.targetsecure.co.uk/newtc24/default.asp?vendor=&MenuId=766&Prod=MBMSI%2DP35%2DNEO%2DF+

I'm only overclocking the cpu, using an Noctua NH-U12P

Will the m/b heatsinks cope with the cpu at around 3GHz?

Your results are going to vary to an unmeasurable extent really. You can set two systems up with identical components and get two totally different outcomes.

With that said, as long as your RAM can support the overclocked frequency or your motherboard can supplement a ratio where by your RAM isn't effected/isn't effected to the fullest extent and heat dissipation isn't an issue you should be okay.

That heatsink appears to have roughly a 34% (at max fan RPM) increase in cooling capabilities compared to the stock Intel heatsink if that means anything to you at all.