How to read from stdin in process substitution? [duplicate]

This reads from stdin:

echo foo | tee >(read line </dev/stdin; echo "internal $line")

You have to keep in mind that a process substitution acts "like" a file.
It could be used where a file is expected. The command tee expects to write to a file.
In that command we are being specific about the device to read from with: /dev/stdin. In that simple example, the /dev/stdin could be removed and that will work also:

echo foo | tee >(read line; echo "internal $line")

If I am understanding your need correctly, this will work:

$ echo foo | tee >(read a </dev/stdin; echo "a is $a") \
>(read b </dev/stdin; echo "b is $b") \
>(read c </dev/stdin; echo "c is $c")
foo
a is foo
c is foo
b is foo

I omitted the PS2 prompt to reduce confusion. Note that each Process Substitution replaces the use of a file (as: tee FILE FILE ....).

The read does not have to be the first command.

$ echo foo > >(echo "hello" | read b; read a </dev/stdin; echo "a is $a") 
a is foo

Note that here the "Process Substitution" needs a redirection,
that is the reason of the two > >( idiom.

A simple echo, will only print the number of the fd used (the name of the file):

$ echo >(echo "hello")
/dev/fd/63
hello

It is similar to:

$ echo "$file"
filename

Whereas this is a very different idiom:

$ echo > "$file"