Difficulty getting a Dell 9020 to cope with 3 monitors

I have a Dell 9020 desktop with integrated Intel 4600 graphics.  I was running it with three 1920 x 1200 monitors with no problems (two on DP, one VGA).

I replaced one of the monitors with a Dell U3011 monitor, native resolution 2560x1600.  I literally unplugged one of the DP-connected 1920x1200 monitors, plugged in the 3011, and I was running with three monitors again, so the configuration was now:

    1920 x 1200  HP e24i

    2560 x 1600  Dell U3011

    1920 x 1200  Dell U2412 

A couple of days later, I was working away and the U3011 and e24i both went black, and all my opened windows moved over to the one remaining screen.  When I looked at the Windows 10 display settings, the two inactive monitors showed as small boxes and repositioning them wasn't possible.  To all intents and purposes, I was running on a one-monitor system.  Rebooting didn't help.

By trial and error unplugging and plugging, I got 2 monitors running, but as soon as I plugged in the third, one of the other 2 would shut down.  The inactive monitor shows in Speccy as "Disabled".

So my questions:

Have I run into a limitation of the 4600 graphics?  Why then did it work to begin with?

I'm running a similar configuration:

    HP ZR2440w (1920x1200@59Hz)
    DELL U2412M (1920x1200@59Hz)
    DELL U3011 (2560x1600@60Hz)

on a Dell 8700 with NVIDIA GeForce GTX 745 graphics with no issues.  The 3011 is connected with a DVI cable.