Absolutely NO POOL chlorine!!
It is very different chlorine than the chlorine used on potable water systems. It is dangerous stuff humnas and animals. Actually nothing but 70% chlorine 1 gram FDA approved chlorine pellets should be used but many people mistakenly think "chlorine's chlorine and more is always better". Which is wrong but look what we have going on here in this thread...
Anybody, but especially a well guy!! using 3" tablets or ANY other pool chlorine is asking for serious law suits. Man that's dangerous stuff you're doing to the groundwater and the people using the water from that and surrounding wells.
When you shock chlorinate a well etc. you run the risk of creating DPBs (disinfection by-products) called THMs (trihalomethanes) which are known carcinogens and limited to 50-80 ppB(illion) by all states and the EPA.
You also run the risk of creating pump, drop pipe, power cable and water quality problems when shocking a well. Most if not all are expensive to fix.
Shocking usually fails and is only a temporary 'fix' if it gets you a Coliform bacteria free test result. Usually in 1-6 weeks the contamination returns.
The proper volume of chlorine is 200-500 ppm of chlorine but there has to be 2-5 ppm of Free chlorine. That is NOT total chlorine, it is the amount that still has disinfection and oxidizing capabilities.
To kill bacteria, the water should be lower in pH than higher, and best is like 6.2 pH. When you chlorinate water it raises the pH of the water which decreases the ability of the chlorine to disinfect but makes it better at oxidizing. You must kill/disinfect bacteria. You oxidize iron etc. and you usually don't shock a well because there's iron in the water.
Shocking a well can cause recovery problems of the well; reduced production. Bacteria form slime to protect themselves from chlorine. The slime can not be penetrated by chlorine and the slime forms encrustations that require acid and/or caustics to remove. That is called well rehabilitation or cleaning. It is expensive.
Rather than allowing the chlorinated water to sit still in plumbing (and the well) for hours, it is best to flush the plumbing every 20-30 minutes and clear the lines of dirt and bring in new chlorine to work better than the older spent chlorine. You flush by drawing water from each faucets for 10-30 seconds and flush toilets and do the tubs too. Do cold water only. Repeat every 20-30 minutes for a few hours. Every hour or so run water down back the well too, for say 5 minutes each run. The chlorine will lay on the bottom of the well unless you do that.