This is just wrong.
I had 0.5cm initial gap and I am on the lower end. Most other people I have spoken to got 0.6-0.8cm initial gap. No one suffers nonunion even remotely. In fact we can‘t ever take a day off because we would risk preconsolidation. One guy even got preconsolidation at 3cm because he slowed down too fast.
The initial gap is to get a head start off the callous considering you start lengthening ~7 days post surgery.
What is wrong? The initial trauma is about 2-3 mm. At max you can take 5-6mm at once and don't risk non union.
If the amounts you mentioned were real and you could take 0.8mm at once with no problem then doctors would fix discrepancies of 1cm with just one surgery and trauma nail. But they don"t because non union is almost sure for 0.8mm and more.
Of course you can have preconsolidation at 3cm if you lengthen too slow.
But you can't lengthen 3cm at once because you will just have a bone gap forever. Thats what the user Hatch asked, how much you can add at once with a TRAUMA nail and not an LL nail. The preconsolidation that can happen is irrelevant because the initial bone gap after surgery is usually about 2-3mm and less than 0,5cm most of the times. 0,7mm will lead to very slow healing and even non union some times. But the problem is not if you can add 0,5 ore 0,7cm at once but 2-3 cm.
Anyways, to answer clearly you can't add 2-3cm at once. The max you can take without risk is 0,5cm and thats to fix only mild discrepancies, not for cosmetic LL. In my case I have about 0.3mm initial gap, I lengthened at a rate of 0,7mm per day as average and never had a problem with preconsolidation.