css – min height by number of lines

Why are you so opposed to the idea of setting min-height in ems? If you have line-height set in ems, multiply that by three and you got your desired height!

div {
    line-height:1.5em;
    min-height:4.5em;
    float:left;
    width:33%;/*close enough*/
}

Fiddled

Update: setting min-height to three lines is an exercise in futility – it does not account for scenarios where content does not fit into the space available. You could use faux columns instead to make content appear to be of uniform height within the row