I just tried to connect three external monitors to my ThinkPad W550s and I get the same result -- only two external monitors and the laptop display receive a signal.

The dock is a ThinkPad UltraDock 40A2.

I tried various combinations of HDMI, DP and DVI connections. That doesn't appear to make a difference.

For the W550s, I found a Lenovo document here that makes it very clear that it supports 3 monitors in any combination, so 3 external monitors should work.

Are there steps I'm not aware of?

I eventually did get an X270 and I am running it with three external monitors off an Ultra Dock (one VGA, one DVI, one DisplayPort) with the laptop closed.

I believe it may be partially a CPU thing - the integrated graphics in some intel CPUs may only support two display. Check the specs for your CPU. My x270 has an i5-7300U and it works fine.

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I eventually did get an X270 and I am running it with three external monitors off an Ultra Dock (one VGA, one DVI, one DisplayPort) with the laptop closed.

 

I believe it may be partially a CPU thing - the integrated graphics in some intel CPUs may only support two display. Check the specs for your CPU. My x270 has an i5-7300U and it works fine.

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Thank you for that info. Your confirmation that it will work is very helpful. I'll forge ahead.

My X270 has the following specs (see below). WIth a Core i7 CPU, I don't think that is my issue.

Could it be the wattage of the adapter connected to the dock?

Or could it be that one of the 3 external monitors must be VGA? I can't imagine that is true. VGA is the only connection I have not tried, given that it is so old. Our monitors are all HDMI or DP. To use DVI, I had to attach a DVI-to-HDMI adapter.

I tested with HDMI + DP and a mix of HDMI, DP and DVI.Changing connections did not make a difference.

I tried disabling the laptop monitor in the OS settings and I tried it with the laptop lid closed. In both cases, I end up with only two active external displays. The 3rd one never gets a signal.

Any further thoughts on this?

Yeah, one of them has to be VGA. The first DisplayPort and the HDMI port are "paired" together, so that you can only use one of those two. Then the other DisplayPort and DVI port are also paired, so again it's one or the other. Then the VGA port is on it's own.