add lab3 exercises

lab3/README.md

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+# CPS Lab 3 Exercise (2021-12-22)
+
+- Mariano Sciacco
+- Federico Carboni
+- Giuseppe Rossano
+- Alessandro Benetti
+
+## Notes
+
+> Note 1: requires `gnuplot` and `phaverlite` (see https://www.cs.unipr.it/~zaffanella/PPLite/PHAVerLite)
+
+> Note 2: all credits to `phaver` executable in this folder goes to Enea Zaffanella
+
+> Note 3: execute `./phaver <filename.pha>` to compile the `.pha` file
+
+> Note 4: execute `gnuplot --persist -c phaverplot.gp filename.txt` to generate the graph
+
+> Note 5: results of the plot are inside the respective folders as pdf files.