In the season three premiere, we see Randall speaking to Toby about going to see the mystery "her." It's possible Fogelman is trying to pull a quick one on us and convince us they are speaking about Kate but ultimately reveal it's someone else, but it also is very likely they could be speaking about Kate.
After Toby went cold turkey on his antidepressants and flushed them down the toilet in an effort to increase their chances of conception in the premiere, producers are potentially hinting at some rough patches ahead. Given that Toby has been on this medication for six years, it's a huge risk for him to immediately stop taking the pills. It's advised to wean yourself off the medication, if recommended by the doctor, and eventually stop taking them, but Toby completely cutting himself off could end up putting their marriage at stake.
In the flash-forward, Toby is visibly depressed and struggling to get himself up to visit "her." Only time will tell if he and Kate have a successful IVF experience and they are able to conceive, but if Toby and Randall are talking about Kate, we just hope it's not due to a health scare from the risks of IVF.
