I had a few issues adding a third pig to bonded pigs, but it blew over after a few weeks. It just takes longer for some pigs to accept that there's another piggy around.
When I got Lottie, I expected Charlie to have to have a problem with her being around, and I expected Tuppy to accept her. It was the other way around - Charlie loved her and Tuppy loathed her. Tuppy lunged at Lottie all the time, bit her and would not leave her alone, so for Lottie's sake I separated them.
For almost 2 weeks, Lottie had separate floor time to the other two. They could see each other through the cage bars and sniff one another. At first they all snorted at each other, but after 2 weeks they simply acknowledged each other and walked away. So I tried them together after giving butt baths to them (not together) and they got on great.