Thoughts: With three blocks – one block must be attached to the other two. Not ready to tackle that . . . yet.

Just going to go for two block connections. 74 varieties excluding rotations.

  My labeling:

Red     Blue   Yellow

 ___     ___    ____

|1 2|   |a b|   I  II

|3 4|   |c d|  III_IV

|5 6|   |e f|

|7_8|   |g_h|

Red to Blue:

One connection (4)

1 – a

1 – b

2 – a

2 – b

Two connections (8)

1 2 – a b (respectively)

    – a c

    – c e

    – e g

1 3 – a b

    - a c

3 5 – a b

5 7 – a b

Three connections (2)

2 4 6 – c e g

4 6 8 – a c e

Four connections (5)

2 4 6 8 - a c e g

1 2 3 4 – d c b a

        - b d a c

        - d f c e

        - f h e g

Five connections (0)

Six connections (1)

1 2 3 4 5 6 – c d e f g h

Seven connections (0)

Eight connections (1)

1 2 3 4 5 6 7 8 – a b c d e f g h

Now double the total since either the red or the blue block can be on top. 2(4 + 8 + 2 + 5 + 0 + 1 + 0 + 1) = 42

Yellow to Blue (or red)

One connection (2)

1 - I

2 – I

Two connections (4)

1 2 – I II

1 3 – I II

3 5 – I II

5 7 – I II

Three connections (0)

Four connections (2)

1 2 3 4 – I II III IV

3 4 5 6 – I II III IV

Now double the total for top/bottom then double for red or blue connection to yellow 4(2 + 4 + 0 + 2) = 32