3" to 8" in 2 months is possible with great forage available but pushing it, especially for Michigan. It is perplexing how a 16" LMB and a 12" got into your pond if it has no inlet out outlet of water connecting up to it... Perhaps you have a neighbor who thinks they are being helpful? This is all speculation, as we'll never know for sure... It may very well be your 8" bass is just a fast grower of your 3" bass or it may be offspring of your 16" bass and other unknown bass that spawned previous year without your knowledge. How large is your pond? Do you have a reasonable sized area that is flat without obstruction? If so, you may be able to do a good seine survey and get a better idea as to just what you have going on in your pond...