I understood!
I made changes in two places.

mainnav.php
<div class="t3-navbar navbar-collapse">
<jdoc:include type="<?php echo $this->getParam('navigation_type', 'megamenu') ?>" name="<?php echo $this->getParam('mm_type', 'mainmenu') ?>" />
</div>

bootstrap.css
.nav > li {
position: relative;
display: inline-block;
}

Now everything works perfectly.
The subject can be closed.
If to somebody helped, write about it please.

Congratulation, Maxim

Feel free to raise question if you have trouble .

Gardner