I am getting a check engine message of cylendar 3 error. How do i know which spark plug is plug 3?

Could be low compression on that cylinder. Do a compression check. Also see if there a vacuum source connected to that cylinder that may cause a miss.

4 spark plugs.

will not be necessary to change those plugs as they will be platinum plugs because the manufacturer knows how difficult to get to them and fitted best plugs that never need changing
besides that , plugs in EFI engine will last the life of the engine and very seldom need changing
most problems are from bad injectors
no need to do a tune up as the ECM does that every minute it is running and if you have problems run the fault codes and fix what comes up

I would start with a compression test on all cylendar. If one is low, then you have a problem there. If all same, then good news. If you have a distributor, these can crack, and let spark go to ground on one cylendar, but show

Hi and welcome to FixYa!
If that's the case then the fuel pump needs replacement. Once replaced you'll be good.

Hope this helps and thank you for using FixYa!

Can be different reasons, but most likely would be spark plugs. Here are some things to check:
1. Spark plugs are usually the first thing to check when you lack power
2. Spark plug leads - if you've replaced the sparks and still no luck, consider replacing the leads as well.
3. Fuel pump - you may not get enough fuel to the engine
4. O2 sensor - connect a diagnostic tool and see if any errors come up
Thank you for using Fixya!

Your 306 code means you have a misfire on cylender 6. That may be the cause of the po 125. Inspecdt the coil, spark plug wire, and spark plug on cylender 6. With the age of the car I would replace all 3 items. If you want know how to test all 3 I can tell you process of elimination is the way to do it. Do you know how to find cylender 6

For the 1996 Mazda 626 2.0L DOHC In-Line 4-Cylinder:

Firing Order:
1 - 3 - 4 - 2

Cylinders:
.........(back).........
|..1....2....3....4...|
........(front)

Distributor:
__________
/////////////////| <---2
_______________
//////////////////////////| <------1
___________
///////////////////| <----C
_______________
//////////////////////////| <-------3
__________
/////////////////| <----4

Let me know if this helped and thanks for using Fixya!

Check the ingition switch I believe green/red approx 9v in the on should jump to 12v in start

Hi,welcome to fix ya.
try disconnecting the coils one at a time and noting the drop in rpm of the engine. if one drops less then the others or not at all replace that coil. If they drop the same on all cylendars then adjust the valves .The exhaust valves get tight and can cause missfire as well.