/
|   |
  /

/  /
|   |   |
  /  /

   /
  |   |
 /  /
|   |
  /

/
  |   |
 /  /
|   |   |
  /  /

Side length 3:
     /
    |   |
   /  /
  |   |   |
 /  /  /
|   |   |   |
  /  /  /

Side length 4:
       /
      |   |
     /  /
    |   |   |
   /  /  /
  |   |   |   |
 /  /  /  /
|   |   |   |   |
  /  /  /  /
(This triangle isn't really filled, the borders make it look like there is a hexagon in the middle, but there is not.)

Side length 5:
         /
        |   |
       /  /
      |   |   |
     /  /  /
    |   |   |   |
   /  /      /
  |   |       |   |
 /  /  /  /  /
|   |   |   |   |   |
  /  /  /  /  /

Side length 6:
           /
          |   |
         /  /
        |   |   |
       /  /  /
      |   |   |   |
     /  /      /
    |   |       |   |
   /  /          /
  |   |           |   |
 /  /  /  /  /  /
|   |   |   |   |   |   |
  /  /  /  /  /  /