It dosent have any mispelings.

I'm pretty sure OP speaks English as a second language.

My first thought was, can you flush the buffer?

A PTS is a single character device. Writing to it causes output to appear on the terminal buffer, reading from it reads from the input buffer. So, writing to it like you do from a separate shell effectively does the same as calling print() from python which has it as inherited stdio. There is a way to write to a PTS input buffer but it's not straightforward and works in a completely different way. Use something like tmux instead, or better, sockets.

Just guessing here but could it be because you haven't set up correctly pt2 as stdin for pt3, try to invoke the command as

script.py 

This is because the "tty" (by which I mean the device named by the output of ´tty´) is only displaying what is sent to it. Be it from the keyboard or pty2.

The fact that the keyboard also fills an input buffer from python has to do with how python and the keyboard are attached to the same input file device which is a separate thing from them having same output file device.

If anything that could output to tty2 could inject inputs to something using tty2 as an input buffer, that would be a security nightmare.

Now, I'll sit back and let Cunningham's law kick in.

With the correct permission you should be able to write straight into pythons stdin from /proc like

Cat whatever > /proc/$pythonpid/fd/0

Named pipes are your friends. Or sockets.

Time to dive into the wide world of VTTYs. I believe you’re writing to pts3’s output buffer - writing to its input buffer is a different virtual device / may be a process descriptor, I’m not sure. Look into the history of vttys and it should start to make sense.