4.5 Percussion - adding 3 instruments at once

In the new 4.5, I want to add three things at once, e.g. bass drum, acoustic snare and crash. In earlier versions, I simply added them one at a time, (with the result as per the accented notes the attachment). However in 4.5, I can add the bass drum and the snare, but when I add the crash, it removes the snare and substitutes the crash. I'm not able to add more than two notes and, at the moment, I'm having to do a torturous workaround involving manually selecting Voice 3. What am I doing wrong? Why won't it let me add more three notes on the same beat like it did before?