Okay then. I'd do no more than an eighth. And make sure you're in a good set and setting. Mushrooms are much more "deep" and "serious" than either of the other two substances you've taken. And also much more emotionally intense. In my opinion mushrooms are the most difficult psychedelic, but also the most rewarding. Every mushroom trip I've had that wasn't a low dose involved feeling both the most amazing I've ever felt and the mosy horrible, sometimes in the same moment.
My first trip was on a half eighth (of very good mushrooms though). But it remains the most powerful experience of my life, 7 years later and after tripping many, many more times on dozens of chemicals and much higher mushroom doses.
Mushrooms will completely overwhelm you and will take you along for the ride. There is no controlling them. MAKE SURE you're in a place where you feel comfortable, are with people you are TOTALLY comfortable with (very important), and have nothing that could come up to surprise you (having to leave, talk to someone who can't know, etc). If any of these factors are not met, I can almost assure you you'll have a difficult time. If you pay attention to set and setting though, you will probably end up having your world blown apart in a horrible and glorious burst of psychedelia.
Please don't think of mushrooms as a recreational drug... most bad trips seem to come from that attitude. Think of it as an investment of your time and energy. It WILL probably be difficult and you'll probably think to yourself "why did I do this" on a variety of occasions during the trip, especially the come-up. But it will be worth it in the end if you go into it with the right attitude.
The come-up will be the hardest part... mushroom come-ups usually make me feel dirty and cracked out and strange and uncomfortable and frankly a bit like a drugged-out loser. But just realize it's the process of your ego making room for the mushroom, and it will pass. Just let go. if you give in to the anxiety of the come-up, it might stay with you throughout the trip, but if you let it go, you'll realize it's an illusion.