I heard somewhere on the internet about Beta missions given from Darkel in GTA3. Here is a full list.

1.Child's Play - This mission was meant to blow up a School Bus full of high schoolers.

2.I Scream, You Scream - You had to lure kids to an Ice Cream Van and then blow it up. This mission is given by El Burro.

3.School's Out - The target was to blow up a highschool building. Most likely the Old School Hall in Chinatown.

4.RAMPAGE!! - Blow up 13 cars. This mission was possibly rampage-like like Trial By Fire or Uzi Money

5.Trial By Fire - The objective was to burn people alive with a Flame Thrower. This mission is given to El Burro now.

6.Stadium Flight Fright - You had to bomb the Stadium on Staunton Island with a Dodo because Darkel's favorite football team has lost the game. The airport was located in Staunton Island in the beta.

7.Flight Delay -  Darkel ordered you to bomb the airport with a Dodo. Most likely it was the terminal you had to blow up.

8.Love Hurts - To fly a Dodo into the Love Media building to destroy it, killing Donald Love.

Curtly's Missions

Additionaly I heard about missions given from Curtly. There were four missions:

1.Give it Back -  A thief stole Curtly's package, so he wants you to kill him and get his package back.

2.Head Radio Protection - In this mission you had to protect the Head Radio DJ Novy from a mafia attack. This mission was possibly given after Sayonara Salvatore or the mafia was the Forelli Family. Notice that Novy isn't the DJ of the station now.

Salvatore's yakuza visit-GTAIII-Beta

The mission as seen on a trailer.

3.Salvatore's Yakuza Visit - I don't like this mission. The Yakuza somehow got in Portland and wants to pay Salvatore Leone a visit. You had to go to Sal's house and kill at least 4 Yakuza gangsters as seen on a screenshot.

4.So Long Darkel - Curtly is sick of Darkel's terrorist actions, so he wants you to kill Darkel. R.I.H. Darkel.