Therefore there are surface plates. You only need one that is prooven/known/believed accurate (usally way more accurate than needeed....) then you use that to spot the suspect.
Three plate method is known and it works, but it is slow and needs disipline. Having a surface plate as a reference is great. I bough my first surface plate from GB, it cost 29£ plus more to ship here....it is 250*300 mm wide and supricingly level. Important thing is to maintain it true, avoid abrasion (no abrassive paper/lapping/sin a top of surface plate), avoid denting and using it a general work top.
Laping plates needs truing every now and then...after you spot it using a true surface plate it is so much easies. Lapping plates do not need to über straigh just a straight enough for work. Every time you use lapping plate you change it - if you have a habit of using more front of it you wear front down. If you make efort of using alternative parts of it and turn it every now and then it thends to stay more stright.
Just don't get too paranoid about the accuracy with surface plate it is easy to spot innaccuracy tht has no consequence.