Hi Garry,
A focus on two or three main keywords on your page is perfectly appropriate, even for longer tail keywords. It may be recommended to use longer tailed keywords for pages on your site appropriate for such keywords. As well, it is recommended to put the keyword(s) as close to the beginning of each tag as possible. Keywords can be combined to create longer tail keywords, but only if they are relevant to each other; try to put two unrelated phrases together will look like spam. If your business is located in a certain area, using long tail keywords with location can help your results; with other keywords, your site can still rank for longer tail keyword due to natural relevancy. With a focus of more than three keywords, your tags will be bulky and read unnaturally.