I don't know, as I don't have that configuration but I will take a stab at it. Maybe you can work out a different way to make this work.
HDMI is a purely digital output, while VGA is an analogue standard. Therefore a passive HDMI to VGA cable will not work on a normal HDMI socket. The VGA monitor can't read the digital output. In order for this to work, the HDMI to VGA "cable" needs to contain digital to analogue conversion circuitry, usually an in line box which may be powered by the HDMI socket itself, or need additional power. I've never used one, but from what I hear, these active conversion adapters aren't always good quality. If there is any way to avoid using one, I think it would be a better solution.
DVI-I on the other hand (you have two of these ports) output both digital and analogue VGA compatible signals. DVI to VGA adapters are very common and usually work perfectly.
I guess the third monitor is VGA only? Do any of your other monitors support digital input (DVI or HDMI) or are they all VGA only? If any of them do use a digital input, you might be better off using a DVI port (with DVI-VGA adaptor) for the VGA monitor(s), and use the HDMI port with a HDMI-DVI cable for a monitor that has a DVI connector.