Interrupt triggered only once

Hello Piotr!

1. your timer will never start counting because you put as period, (ARR) zero.

htim3.Init.Period = 0;

2. Inside your Timer callback function(if this finaly called), you zeroize the variable by

defining static int delay_us = 0; finaly after exit it has the value 0x
3 Other issue is that you put inside this callback
the 
HAL_TIM_IRQHandler(..); 
This is not correct because the callback function called from inside HAL_TIM_IRQHandler(..); !! 
4. Inside your While(1) loop, you call sequentialy 
HAL_UART_Transmit_IT(..) without checking for completion from the previous transmit This is a non blocking function. May be calling the blocking version is more suitable for your project 
HAL_UART_Transmit(..) 
In conclusion, you first take action to improove these imperfections in order to get a better focused picture to the posible problem.

It would help the community and you to post your .IOC file.