Yes I have all three, iPhone 3GS, MacBook 13" aluminium and an iPad.
Do I really need all three, probably not. The iPad was a must buy gadget, but it has become my primary bit of kit. The iPhone has pretty much been reduced to just being a phone, once I played Angry Birds on the iPad I couldn't go back to the iPhone. Same for FB,twitter and email.
Feel sorry for my MacBook it's been neglected and we were so close, but times change

the MacBook is my store for music, photos. I use it to rip and burn. And it holds my material for work. I teach at home so the MacBook has the audio tracks and print outs etc.
It's the battery on the iPad that is so amazing, I can surf for ages sitting on the bed or sofa. Do still take the MacBook to bed to watch BBC iplayer, I have to use a VPN and openvpn isn't available for the iPad. (pptp VPN on the iPad can't hack the distance to the UK). So in this respect I feel like the head and shoulders ad, " still taking a shampoo and conditioner in to the shower"
If I had the cash, I think I would readjust my line up, losing the MacBook adding an iMac, I could watch iplayer on a bigger screen. As for the iPhone well I won't be upgrading to iPhone 4, can't justify it. Might go to iPhone 5 but not in a hurry.
As I said in a previous post I hope in future to see an iPad-pro that merges more of the MacBook Air into the iPad. Hook up a big screen, keyboard and mouse when needed, unhook to lounge around the house or the coffee shop
There is a lot of overlap in the products, but something for everyone.
Different strokes for different folks.