They both have micro usb ports but i thought I could use my current galaxy s2 cables with the tablet and they don't fit. Are there different sizes and how can you tell when getting a cable?
Watch out for this, yes there are different sized cables they look the same but aren't. You will probably be able to squeeze it in and it will work but may break the connector after a while(this happened to my Galaxy Note 2). It may have something to do with different pins within the cable itself.
I found it very hard to tell the difference but try to just use the one that came with the device. If not just use a different cable but from a newer product as there are definitely different pins within the cable. Best thing to do is if a cable doesn't fit in easily don't put it in.
It should easily go in. The ports on the two devices are the same and with same orientation of the port as well. Maybe there is something stuck inside the port on your Galaxy Tab 3. You can usually tell by the shape of the male port. The two slanted ridges on the size are an identifying feature of Micro USB A/B. B also has two little latches on the bottom. Micro USB standards A and B are compliant with each other as well but these two devices should have the exact same Micro USB B Port.