Lintels (3-unit pieces) must sit directly upon exactly two units of the player's color. (They must also be elevated by stones at both ends.)
So placing a lintel so that it sits upon three of your units, as in the situation you described, is illegal.
Interesting; the game description here says:
(3x1) rectangles (lintels) which can be placed in such a way that they are supported by two blocks of your color. The simplest configuration therefore resembles a mini Stone Henge (hence the name).
The space in between can be empty or occupied by a piece of either color.
(which interpreted literally implies covering 3 of your own is OK, because if 3 of your color support it, then certainly 2 of your color support it. It's also not so explicit that both ends must be supported.)
but the rules at http://www.gamerz.net/pbmserv/druid.html say
Lintels must lie flat, be supported at both ends, and rest on exactly two units of same-coloured stone.
It seems the gamerz.net version is more precise and trustworthy; I'll edit the BGG wiki description...