In my endless endeavours to provide my citizens with ever shorter transfers whilst preserving as much real estate as possible, I found a few tricks :

• Up to 4 metro stations can be squeezed into a 10u road by having their entrances face the road sideways. Even with the 90 degree angle, people will still use them !
  
• Metro entrances will join together if they are placed close together (within 1 tile), allowing for people to transfer directly underground !
  
• You can build a road 1 tile inside a metro entrance, allowing people to enter and exit the station through that road.

Also, these configurations are all created without mods, just use some temporary roads to move the stations in the position you want.

This first variant seems the most real estate and pathing efficient :
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Underground view :
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The bottom 2 stations have a combined entry and exit in the middle one, facing north, with the northern station exits to the south, thus providing a very short transfer distance. In this screenshot I put a building between the two, but I suppose you can also put a footpath or road there.

VARIANT 2A

Sometimes the bottom to stations can't be positioned like Variant 1 (because they overlap) so for those situations this variant works :
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Underground view :
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Note that the road must end 1 tile inside the second station !

VARIANT 2B

In the variant above people sometimes won't enter the upper station, so in that case you can change it to this :
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Underground view :
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Note that the road must end 1 tile inside the second station !

You can also combine the entrance of 3 stations, like this :
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Underground :

If needed, you can even put a road between the 3 combined stations, and people still move between them (but pathing is less optimal than having them all on the same side of the road) :
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Underground :

Here's a variant I use for a big transport hub in my city. Note that even though some stations do not have road access, they still work perfectly, because the station with the final shared entrance (the one that everyone uses for all stations) does have road access !

A few more tricks are included in this as well :

• Bus terminals and monuments can be 1 tile away from the road, allowing for e.g. pedestrian tunnels before them.
  
• Pedestrian paths can be connected to the right middle side of bus terminals, allowing people to shortcut.
  
• You can connect a pedestrian path to the back of a railway station causing passengers to have a quick way to the other side.

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Underground view :