I ran into a somewhat similar situation recently where I was tempted to use "both" in reference to three items, as in:
I did see a notable increase in my approach frequency when placed into a rare situation in my city where there was both high traffic overall, a high fraction of the target demographic, and conditions favourable to approaching.
For this case, I found that "together" did the trick nicely:
I did see a notable increase in my approach frequency when placed into a rare situation in my city where there was together high traffic overall, a high fraction of the target demographic, and conditions favourable to approaching.
This doesn't work quite as nicely in your particular example, but I thought I'd mention it since it works well in some cases.
It does work for your example also, just not quite as nicely:
There are several recommendations I have to further improve the sites — together to improve their profit, decrease their cost and improve their usability.
However, for your example, some of the other suggestions, such as simply removing the conjunction altogether, are probably just as well. "Together" was more crucial to my example because the simultaneity of the factors was more significant to the meaning of the sentence.