No, you can be a guarantor to as many people as you want.
I would thoroughly recommend against you being a guarantor - a finance company have looked at their situation (including their past financial history) and do not believe that they will pay.
You do not have access to the same information that they have and so are making a decision based on friendship rather than facts.
3000 over 6 years is (in my opinion) not a very difficult loan to get - if they can't achieve this by themselves then there would be presumably a detrimental credit history to their name.
Please also note that it is usually not just a guarantee than you are signing but a 'guarantee and indemnity' - which goes further. It means that you indemnify the finance company to ALL their costs - ie not just the loan, but any other costs that they incur in enforcing their agreement (eg tracing agents, recovery agents, court action against them etc)