There's three things: is it correct? [duplicate]

The combination of there's with a plural noun is common in informal English. This is particularly the case in spoken language, in which there's rolls off the tongue more easily than there are.

The usage should be avoided in formal English, however. And note that is not permissible to use the uncontracted form:

*There is three things to know about ... .

Here is an extract from the Cambridge Dictionary's page on There is, there's and there are:

In speaking and in some informal writing, we use there’s even when it refers to more than one. This use could be considered incorrect in formal writing or in an examination:

• There’s three other people who are still to come.

• There’s lots of cars in the car park.

http://dictionary.cambridge.org/grammar/british-grammar/there-is-there-s-and-there-are