Well normally, you just float em all left, that causes them to align next to each other.
The vast majority of themes should already have image positioning classes built in
Take twentyten for example:
.alignleft,
img.alignleft {
display: inline;
float: left;
margin-right: 24px;
margin-top: 4px;
}
So assigning 3 pictures the class of alignleft would cause them to line up next to each other horizontally (so long as the width of all 3 doesn’t exceed the width of the container element…..)