Last week I've bought a Raspberry pi3 b+ for my project. Today I've accidentally connected the 5v pin and the other 5v pin or with the 3v and then it has just shutdown. This time it wont boot, only red light and never the green one. Is it burnt? I've also tried to format the sd and rewrite a new os, but nothing happened
Connecting pins at different voltages will typically destroy your Pi. That inclues 5V to 3V3, 5V to GND, 3V3 to GND. Connecting pins the same voltage, e.g. 5V to 5V, will not matter because they are connected internally anyway.
Connecting any of these power pins (5V, 3V3, or GND) to a GPIO pin may also kill your Pi, depending on the mode that the GPIO is in and its state. If it is an output set to LOW, then its a GND level, and connecting either 5V or 3V3 to it will create a short circuit. Likewise an output set to HIGH (which is 3V3 on the Pi), will create a problem when connected to 5V or GND. If the GPIO is set to input, then it should survive GND or 3V3. It's still not rated for 5V.
You said that you had done this a few times. There is a polyfuse on your model of Pi that can protect you sometimes. If you did this more than once, you may have damaged the fuse to the point that it could not protect you. It's also possible, even with a new fuse, to burn it out. The fuse is not a perfect protection. There are several posts here an elsewhere about the resilience of the fuse and its time to reset.
Connecting a 5V pin to the other 5V pin will not do any harm.
The 5V pins are connected to each other internally and form the 5V power rail.
The 3V3 pins are connected to each other internally and form the 3V3 power rail.
The ground pins are connected to each other internally and form the ground rail.
Connecting a 5V pin to a 3V3 pin will destroy the Pi.