I changed the water in my tub 2 weeks ago. Today I bought a LaMott3 4-way test kit and tested the water. The tub is a canspa with ozonator and holds 350 gal.

With hth strips my readings are :

Br 2-3 ppm
PH 7.4-7.6
Alk 80-120 ppm

With LaMotte 4-way test kit :

Br 2.25 ppm
PH above 8.2
Alk 110 ppm

I also took a PH reading of tap water with booth and got a reading of around 7.4.

Is there any reason why the PH readings are so off in my tub?

soaps, body oils, perfumes, etc... all affect your chemicals. Get used to fluctuating readings, as this is completely normal. Use PH down or dilute muratic acid to bring the ph back down. Check regularly, best if every day.

With hth strips my readings are :

Br 2-3 ppm
PH 7.4-7.6
Alk 80-120 ppm

With LaMotte 4-way test kit :

Br 2.25 ppm
PH above 8.2
Alk 110 ppm

These PH readings were taken about 3 mins. apart. I can't see the PH jump up to over 8.2 in 3 mins or can it?

you are using two different test methods. There can and usually is variance with different methods at different levels. Test strips are not as reliable as liquids.

How reliable are electronic ph testers ? Worth the cost ?

Am thinking of getting one once my test kit runs out soon. TIA

With electronic ph testers, you pretty much get what you pay for. You don't have to be that precise with your ph. Generally, one scale above and one below are tolerable ranges when using the drop method.