I made my first IMBC, but wanted to try just half recipe. So I used 3 egg whites (full recipe calls for 5), the eggs were kind of small. Otherwise I followed the recipe. Now it is runny. I am keeping this in the fridge, it is there about 45 minutes, still not quiet spreadable. Should I add some more butter, or PS??? How to make it stiffer?? Thank you

Leave it in the fridge until it get firm, then let it soften until it's just a bit cooler than room temp and beat the HELL out of it with your paddle attachment... if this doesn't save it, then you need to start over.

I find the Beater Blade makes mixing things more thorough, and fluffier with no stopping time to scrape down the sides. Great for beating the hell out of icing.

OK, well, I added some more butter, let it chill, beet the hell out of it...and was able to ice the cake, but it seems to me a bit soft. It will definitely not be good for piping. Will see if the fridge help settle down the cake, it is only for the practice IMBC taste, so not a big deal.
But I still wonder what went wrong. Thank you for your help here!!

Do not add PS to IMBC - it breaks it down.
The butter temperature has to be quite cool for it to be done well . If it s watery leave it for a few hours and then beat it and it should come together.

...OK, and also I notice a little of "water" or some kind of clear liquid comming out of the piping bag??? What was it? I really beat the hell out of it!

Liquid sugar syrup by the sounds of it. Try using the whisk attachment until it all comes together.

Did you get your syrup to the soft-ball stage (230-238 degrees?) I've accidentally added my syrup too early a few times and it wasn't at the right temp yet....had the same situation you're describing.

Yes if your sugar is not cooked to the proper temp it can lead to problems. Once the sugar reaches soft ball stage it is no longer a syrup as the water has all been cooked out of it and is only pure sugar. If you don't cook it to that stage, your IMBC is going to break down.

Thank you...that could be. I don't have the thermometer, so I though I boiled pretty good, BUT "soft ball"? Hm, I am not sure what that should look like...but it was probably the problem. I am going to buy the thermometer for next time. Thank you ALL again, you were very helpful.

A thermometer is a great tool to have and a good investment. I have both an instant read, and a candy thermometer that clips to the side of the pan.

However, I just bought the Cooks Illustrated Magazine American Classics 2009. In the Mile High Lemon Merenge Pie section they deal with boiling the sugar to the right temp. Here's the quote:

"There's only one challenge when making an Italian meringue. Professional bakers use a candy thermometer when heating the sugar syrup to the correct temperature (between 238 and 245 degrees), but many home cooks don't have this tool. Could I make an Italian meringue without a thermometer? After much trial and error, I found that boiling the sugar syrup for exactly four minutes worked perfectly every time - no matter which test kitchen stove or pan I used."