a 90-day Tourist visa? It is the validity time for a single-entry tourist visas. That means you can enter China within the period. Otherwise, your visa is blank out. You can apply it through Chinese Visa Application service Center, the fee is 30 British Pound per person.
You need to provide:
1. Original passport remaining to be valid for at least 6 months from the date of application and with sufficient blank visa pages.
2. One completed Visa Application Form (e-Form / Download) with a recently-taken passport photo;
3. Other documents would be helpful for you to get visas, if there are any.

Erm so that is the validity... what if we would like to be in China for 3 months, is that still the length of the tourist visa we can stay in China?

There are two types of the single-entry L visa to china, One is of 3 months' validity, and the other oof 6 months' validity. The fee is different.

Usually the validity of a Single Entry or Double Entry "L" visa is 90 days or 180 days from the date of issue. This means the holder of the visa shall enter China no later than 90 days or 180 days from the date of issue, otherwise the visa will be expired and is null and void. Normally the duration of stay of a "L" visa is 30 days, which means the holder of the visa may stay in China for up to 30 days from the date of entry. If your estimated stay in China is longer than 30 days, please make it clear in the application form and ask for the Visa Officer's approval when you submit your application..