When the 3000 were converted, IIRC, they were still using the synagogues.Good answer, good answer.
Believers did meet "from house to house" early on. Imagine 3,000 people gathering at homes on Sunday. No wonder there were many elders. They had a lot of meetings to preach to or lead Scripture study.
Church buildings as we know them came later.
And I believe that was a temporary situation. When that many people got converted, of course, the meetings would be that big. But there is no evidence it stayed that way. They then most likely broke up into small groups in people's homes. Hence, so called "megachurches" and large liturgical bodies alike, (which need so much organization, which requires more money and resources to run) are looking at the wrong things when using this scene in Acts as their model of church fellowship.
When I talk about spiritual kinship, I'm not referring to an institution or denomination. I'm referring to groups of believers who have held to NT doctrines and practices. I believe such groups have existed since NT times. (And apparently so do you).
Those groups were imperfect, of course, just as they are today, and just as they were when Peter, Paul and others were writing to them,correcting, instructing and encouraging them.
Still, it is not right to go picking out just any ol' groups to fill in the "gaps", and solely on the basis that they stood out from the Catholics and were persecuted, and then go on and vehemently deny that they were aberrant against all the historical evidence. Sorry, but not all of those named groups tossed around measure up to NT doctrine, as the groups claiming them even hold. "Groups" can be as small as "two or three" (Matt. 18:20). It does not have to be an organized sect, but that's what people tend to look for.