You can share the 5v with more than one device, yes. A boost (not buck, a buck is a step down one) converter will try keep the output voltage at the voltage you configured it to, independent of load. However, you need to make sure your boost converter can handle the amount of current you are going to draw, and the source (3.7v one) should also be able to supply sufficient current. 3.7v sounds like a single cell lithium battery, not sure what kind of Emergency light you have, but it is likely they are not meant for large current draws either, escpecially if that voltage drops to something like 3v. And if you are using a XL6009 or an MT3608, you should probably not try to charge more than one device for example. They are not suited for high input/output currents. Quick calculation: 5V 3A output is 4.05A at 3.7v and 5A at 3V, neglecting efficiency losses.
Simply connecting multiple converters together is not possible either, since there is no way for them to share the load between them, you would need to design a load sharing circuit, which would be interesting to try, only this doesn't change the current the battery has to supply.
Hope this helps.