Shoulder to Shoulder is a 1975 game by David Parlett, published by Intellect Games.
These rules are from David Parlett's website:
The board and set-up. The board
consists of 114 hexagonal cells, 38 each of red, green and blue.
The three central cells, one of each colour, are each marked with a spot and are referred to as home cells.

Three players,
designated Red, Green and Blue, each have twelve pieces of their own
colour. They start by placing eleven of their pieces on the edge
cells of their own colour and the twelfth on the home cell of the
same colour.
Object of the game. Your main aim is
to get all your pieces together into a single connected group, so
that each one lies edge-by-edge with at least one other of your own
colour. The group may form a large area or a line, or a mixture
of the two, so long as all its constituent pieces are connected. In this
example (right), Red has just "connected" and play
immediately ceases.
A secondary object, in case of failure, is to ensure that your own
pieces
form as few connected groups as possible (for which purpose a
single unconnected piece counts as a group). The winner scores zero
penalties
and each opponent scores one penalty for each distinct group formed
by their own pieces. The minimum penalty score is therefore normally
two.
However, a player who at end of play is only one move away from
forming a single connected group scores only one penalty instead of two.
In this example, Green counts three penalty points and Blue
four, while Red counts none.
A match is three games, and the ultimate winner is the player with the lowest penalty score at the end of the third game.
Movement. Red moves first
in the first game, Green in the second, and Blue in the third. The
order of play within in each game is always Red, Green, Blue.
At each turn you move any one of your own pieces. There are two different types of move: the straight and the
squeeze.
A straight move is one that follows a straight line, crossing at
right angles over the edges of adjacent cells. You may move your piece
one
or more spaces across any distance, so long as it does not land on
or jump over another piece of any colour. You may end the move on an
unoccupied cell of any colour except for the home
cell of either of your opponents. You may pass over an opponent's home
cell
if it is vacant, but the only one you can land on is your own.
Three examples of straight moves are shown at top left of the
diagram.
The squeeze is a short move that can only be made
by a piece occupying a cell of your own colour. In this case you can
move
it to any one of the nearest vacant cells of the same colour (your
own), by making it exit from one corner of the cell and travel along a
single edge until it reaches the next cell of the same colour.
This move is called a squeeze because it may be made even when either or
both
of the cells it passes between are occupied by a piece or pieces
of any colour, so that it is forced to squeeze between them. Three
examples
of squeezes are shown in the diagram above. Note how the red piece
on its home cell squeezes between a blue and a green.
Play continues until one player has connected. No more moves may then be made.
The two-player game
This is how the board is set up if only two are playing.
The twelve pieces of the third colour (in this case blue) are arranged as
shown, with two each radiating in all six directions from the home
cell of that colour. The unused home cell is left empty. Follow all th
rules of the three-player game but with these exceptions:
(a) You may not move any pieces of the third colour - their purpose is merely to act as hindrances.
(b)You may not land on or pass over the home cell of the third colour.