I disagree. Regarding 1 and 2: Preterite is used in written German, while the perfect is used in spoken German. Both your sentences are grammatically correct. Without any context it is difficult to comment on it. 3. Sie soll immer zu spät zum Unterricht gekommen sein. That does make sense. That means that someone has heard that she was always late in class, but cannot confirm it as he did not see her being late. Elliott's suggestion: Sie sollte immer zu spät zum Unterricht kommen. That has another meaning: It means that it would be better for her to be late in class (for some reason not given here in this context).

1) Sie hat das gut gekonnt. (Google Translate sagt: "She did that well." Ist das richtig?) Ona je to dobro mogla/uraditi-gut gekonnt proslo vrijeme, koje ne podrazumjeva i danasnje i ovoj recenici. 2) Sie hat das gut machen können. Ona je to mogla dobro napraviti.- još uvijek moze, odnosi se da i danas može 3) Sie soll immer zu spät zum Unterricht gekommen sein. ("She should always be late to classes"? Das macht keinen Sinn. Ona je (navodno) uvijek kasno na predavanje dolazila. samo u proslom vremenu dolazila kasnila na predavanje

1) She could do this well. It means there was a time when she had the skills to do it well. It does not necessarily mean that she did it well, the last time she did it. But of course she may still have the skills. It's hard to say out of context. 2) It was no problem for her to do it. / It didn't take her any effort to do this. / It was easy for her to do it. - This sentence doesn't say anything about the result of whatever she did. We don't know, if she did it well. We only know that doing it didn't cause her any trouble. 3) I've heard she was always late for her lesson. Jemand soll etwas gemacht haben oder machen - That is a very common term when we know something only from hearsay. It's used in connections with rumours and gossip about not too close acquaintances but also celebrities or suspects. Weißt du wie es Paul geht? Ich habe ihn schon lange nicht mehr gesehen. - Oh, er soll jetzt im Ausland leben. XY soll in diesem Film die Hauptrolle spielen. Die Polizei hat den dringend tatverdächtigen Mann festgenommen. Er soll seine Frau ermordet haben.

1. Sie hat das gut gekonnt --> She could (do) that well. Personally, I'd put können in the preterite. Sie konnte das gut (machen/tun). 2. Sie hat das gut machen können. --> Same as number 1. Again, I'd say... Sie konnte das gut machen. 3. Sie soll immer zu spät zum Unterricht gekommen sein. --> She is supposed to have always come late to class. I agree, it doesn't make sense. But again, to streamline this, I'd put sollen in the preterite to simplify gekommen sein... Sie sollte immer zu spät zum Unterricht kommen.