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Both is the suppletive variant of *all two, which is not grammatical English.

Suppletion is the irregular grammatical phenomenon of substituting a different word or root.

Like using went instead of *goed, or ever instead of *anywhen. It's not too common in English, but it occurs.

So the equivalent of both, for N> 2, is All N: all three, all four, all seventy-seven of them.

I would leave out the word altogether, the second example you give makes perfect sense as it stands:

There are several recommendations I have to further improve the sites; to improve their profit, decrease their cost and improve their usability.

Or, if I'm going to be really picky, and remove the doubling up of the phrase "improve":

There are several recommendations I have to further improve the sites' profit, decrease their costs and improve their usability.

You don't need a word there at all.

There are several recommendations I have to further improve the sites — to improve their profitability, decrease their cost and improve their usability.

A follow-up suggestion, which you didn't ask for, is to remove the triple repetition of the word "improve".

I propose several recommendations for the sites to raise their profitability, cut their cost, and improve their usability.

Furthermore, a cost decrease generally implies a profit increase, so that may be redundant.

I propose several recommendations for the sites to improve both their profitability and their usability.

You could try a combination of "both" and "as well as".

"There are several recommendations I have to further improve the sites — both to improve their profit and decrease their cost, as well as improve their usability."

Although, this might put an emphasis on the last point.

There are two usual alternatives to refer to the three items:

1. "not only...., but...and..."


2. "both...and...as well as..."

The first alternative - "not only..., but...and..." - has a limitation: it is basically intended to refer to two items, but is adapted to fit in three items by adding "and...". Moreover, "not only..." puts less emphasis on the first item.

The second alternative - "both...and...as well as..." - puts an emphasis on the last item.

Therefore, the best way to refer to three items is to use "...: firstly...; secondly...; lastly...". In the instant case, the sentence could be written as follows:

"There are several recommendations I have to further improve the sites: firstly, to improve their profit; secondly, to decrease their cost; lastly, to improve their usability."

Logically, you don't have "several" recommendations, but "three".

There are three recommendations I have to further improve the sites - to improve their profit, decrease their cost and improve their usability.

Cheers

Consider triad

A grouping of three.

https://en.wiktionary.org/wiki/triad

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.