Why is Google rewriting the page title with '[3]' in the search results?

It seems to have to do with duplication of words in the Title. I ran a screaming frog of your site and noticed many Titles have the same words repeated. Take a look at this search: https://www.google.co.uk/search?q=designmynight&oq=designmynight+&aqs=chrome..69i57j69i60l5.9033j0j4&sourceid=chrome&ie=UTF-8#q=designmynight+mexican+and+tequila+bars

Some of your pages listed have the [3] in the Title and some do not. Every one that has the [3] has a Title that has repetition...for example "Mexican and Tequila Bars Manchester | Mexican and Tequila Bars in Manchester | DesignMyNight" & "Mezcal Bars London | Mezcal Bars in London | Design My Night"

Not sure why the Google algorithm is adding the [3] in these instances, but definitely it doesn't like your Titles that it sees with duplicated words or stuffed as the [3] doesn't seem to appear on non-stuffed Titles.

The example you gave originally doesn't have the exact duplication issue that I found in some of your other pages, but the algorithm may be a bit finicky as to what it thinks is stuffing.