What happens if I mix 3 DIMMs with different timings?

[leenasked]

I have an old PC which had 4 gb Corsair 1333mhz 9-9-9-24 installed.
Now I wanted to increase the memory so I bought 2 more 4gb ram but they're 1600mhz with 11-11-11-28 timing.

The BIOS says I have installed 12gb memory of 1333mhz with 9-9-9-24 timings.
How do those slower DIMMs are running at faster timings? Is that a problem? If yes, how do I solve it?

My computer boots just fine, works just fine but i'm worried about the stability of the system.

My system:
Intel core i5 2400 (Intel website states i5 2400 supports only upto 1333mhz memory, what should I do now?)
ASUS p8h77-M (Motherboard supports memory upto 2133mhz)

 

[CountMike]

Don't do anything if it's stable, all is just like it should be. Considering that your 1600MHz RAM is now running at 1333MHz, the Cl also dropped to 9. It's a good thing as it's performance is now at or close to what would be at 1600MHz.
In practical use, you'd never see any difference in speed but increased amount of RAM can surely help more.