Hello, until recently my Raspberry pi3 b + worked with OSMC prekasno, but after trying to connect DAC, OSMC stopped loading, I wrote a new image on the memory card but the installation did not come up. There was a rainbow screen, I tried to change memory cards and power supplies but all to no avail . I remembered about Noobs and with it, OSMC was installed on a memory card. Help to understand, other systems are installed well, even without the help of Noobs. I apologize for my English.
A rainbow screen could indicate that you have taken a wrong Image for the specific Pi. Double check the Pi version you own.
This could indicate a faulty SD Card, did you do your second install on a new card?
Yes, I used two new cards, the result was the same. 
Than I assume wrong image used. Take a picture of your Raspberry Pi and post it and let us know which Image you used during install.