Is it safe to combine Juvederm 3 (UK) with Juvederm Ultra (USA)?

You can combine fillers in layers or together if done properly can sometimes have an enhanced look . Best, Dr Emer

Yes, it is fine to combine fillers. I prefer to use a micro cannula to inject the lips to prevent bruising. Look for a cosmetic dermatologist or plastic surgeon who is an expert in fillers. 
I hope this helps.

There is no problem using both. As a matter of fact, the equivalent to Juvederm 3 is Juvederm Ultra.There is no danger in using different fillers for the same area. In lips, Juvederm Ultra will not last as long as Juvederm Ultra Plus (equivalent to Juvederm 4). You can also use the high viscosity Juvederm Voluma for lips as long as it is properly injected. It will last much longer. Other options in the US is the Restylane line of products, also available in the UK. Good luck!