Is there any inyun to not teach a child Alef-Beis inside of a book or siddur before they turn 3?
The Rema (245:8) writes that from the age of three a childn is taught the aleph beis. The source of this ruling (commentary of Abarbanel to Pirkei Avos) explains that this is in order that by the age of five, the child will be ready to study Mikra, as the Mishnah writes. The Mishnah (Avos 5:21) does not mention anything about the age of three.
I do not know of sources that state that one should not teach a child the letters beneath this age, though I don’t see much point in doint so either.