Concatenate 3 columns but triggered when data is placed in a different column

Hello: could someone please help me with a simple macro to:

concatenate column E, I, and G (in that order) with spaces in-between and display the result in column J.
This action is triggered when data is placed in column D.
And this concatenation would apply to rows 3 to 1000

This worked very well, thank you so much. I neglected to include in my requirements, specifically regarding the trigger in column D, that the entry in D will most likely be an entry that is pasted. Is there a way to write the code so the action will be triggered if the entry in column D is typed or pasted?

Thanks for your quick response. I think the code that was provided by jeffreybrown is closest to what I need except for the comment I just included in a reply to him. I do appreciate it though.

Is there a way to change the code so the concatenation will be triggered if data is pasted into column D? Right now the code only works if something is typed into the cell.