Too Much Yeast?

Hi everyone. I am a young and ambitious brewer who dove right into all grain and had a quick question. How much yeast is too much? I started off making 3 gallon batches and will be making my first 5 gallon batch next week. But will 1 tube of liquid yeast pitched into 3 gallons of wort yield the same results as pitching that same amount of yeast into 5 gallons of wort? Will the former simply produce a faster fermentation? The reason I ask is that my first beer (3 gallon batch with 1 tube liquid yeast made with a starter) did taste a bit on the yeasty side. But im pretty sure i bottled it prematurely. However on a different 3 gallon brew I made another yeast starter from a full tube of yeast and pitched about 3/4 of the starter and the beer never reached full attenuation (it was also slightly undercarbonated after bottle conditioning). So, if brewing 3-5 gallons is it always a good idea to use a full tube of yeast? With or without a starter? thanks.

Use a starter when using a liquid yeast on every batch. Check out the brewers friend yeast starter calculator to get the proper starter for the proper pitch rate for your recipe.

http://www.brewersfriend.com/yeast-pitch-rate-and-starter-calculator/

Depending on what you are brewing will allow and if you are willing use a dry yeast like us 05.

1.) You need to use a yeast calculator to determine how much yeast you need to pitch and whether or not you need to make a starter. MrMalty or YeastCalc
2.) It is a very difficult thing to pitch too much yeast. You'd have to pitch a gallon of liquid yeast into 5 gallons of wort to over do it.
3.) If the wort has reached FG and the yeast haven't flocculated just cold crash for a couple days and rack off the yeast.