A 3" diaphragm is typically too big to produce high frequencies efficiently and give even dispersion. Most 3" cone drivers will not play smoothly and evenly all the way to 20khz like a 1.25" dome can. A 3" will typically have poor off-axis response and you will experience "beaming". Many full-range drivers are 3" because that size seems to be the best balance for playing low and high frequencies; but if you have a mid-woofer and you don't need full range capabilties then there is no reason to use a 3" driver for high frequencies.