Speakers on homepage only ever has 2 columns instead of 3 (IE11 only)

[samsmithyeah]

Description of the issue:

The list of speakers on the homepage should be divided into 3 columns (when the browser width is wide enough to accommodate 3). However on IE11 a maximum of 2 columns is reached.

Browser used: IE11
OS used: Windows 7
Device used: Browserstack

Steps to reproduce:

1. To to react.london on IE11
2. Scroll down to 'on stage' section

Actual result:

Expected result:

[chris-brookes]

Re-tested. The issue is still happening.
All of the other browsers change to two and one columns when the page becomes more narrow, but the columns are aligned to the left hand side. When fixing this issue I think it would be good to centralise the columns when they change to one and two columns.