I have strange behavior i flashed newest BF to my newest XJB 145 and everything seems fine when the drone is connected to the usb. When I plugged it out and use only battery there are 3 esc beeps and nothing more. Leds are not flashing also and it won't arm.

I had exactly the same situation with ARFun 95. I broke it, replaced the tower FC and esc and after pluging the battery there was no life in it only 3 beeps. 
Before crash everything was working fine. I realized that the FC could not be powered so I added the power from battery leads to the FC directly and now it is working but as far as I remember there was no such wires before. 

FC led is not on with battery connected. If the 5V regulator is dead can I power it somehow different or now all i can do is buy new one for $30?

This is probably the one that I have: https://www.hglrc.com/collections/flight...k-rc-drone

I forget ... after my IBUS reciever didn't want to work with SBUS pad I had to connect it via RX3 (UART 3) port so I soldered really near the 6R8 (on the picture) so this is the voltage regulator right? As far as I remember i checked everything twice after soldering and all was working well even with battery connected

Strange that this is happening on my second drone after flashing BF. XJB is new so what is happening? Any help will be appriciated

This post is a mess I know but that are quick thoughts that came to me right know.

Read your post if this helps try Joshua bartwell he has a lot of youtube posts and has written and helped create some nice f4 fc's maybe he can help.

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Joshua bardwell

If you re-flash a FC you will need to go through the whole configuration/setup process again in Betaflight.
Ports tab- is Serial Rx on correct UART.
Configuration tab- ESC protocol, board orientation, gyro/PIDs loop speed ect ect and so on so forth.

If you do need a configuration walk-through just let us know.

Did you save a DUMP from your CLI? Whenever I flash a new FW, I always perform a dump first. After the flash, I paste the dump back in and then SAVE. This reboots the new firmware with the old settings. If you have your dump from the last FW, then you do the same.