1) only two targets. Keigo and Azusa. they were surprised to see the targets outside of the park when they had just sent an entire hit squad into the park to kill them.
2) back in chapter 34 he said it to Azusa, pretty obvious there.
3) Pretty sure they were referring to the seniors as adventurers back at the guild but i can't be one hundred percent sure without the original page in japanese.
So, I just managed to find the raw.
1) you're right. It's also obvious from the image. I don't understand why manhwafreak said three targets here
2) he didn't say hello btw. It was よし, so it should be "alright". Basically he was expressing that he was done with his tasks.
3) Miyoshi was referring to Yoshimura. She used senpai to address him. Basically, this dialogue should have been "I'll defeat the zombies. I'll leave the skeleton to you, Senpai."
They have never worked with the other adventurers, so it would be weird if they suddenly got requests from their "seniors".