Q. Dear Rabbi. Does the low seat or stool used during Tisha Beav has to be less that three Tefachim high?
A. A Tefach  according to Rabbi Chaim Naeh is about 8 cm, (3.2 inch.), whereas according to the Chazon Ish, one Tefach equals 9.6 cm. Halichos Shlomo (p. 436) opinion is that it does not necessarily have to be exactly less than 3 Tefachim.
Horav Shlomo Miller’s Shlit’a maintains that on the onset it should be indeed less than three Tefachim, however in need it could be a bit more.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller, Horav Dovid Pam, Horav Aharon Miller and Horav Chanoch Ehrentreu and Horav Kalman Ochs Shlit’a