It looks like this is Madö King Granzört.
mecha anime probably from 90’s
According to the wikipedia page, this aired in NTV from April 7, 1989 to March 2, 1990.
The hero of our story here is a boy called Daichi, who
arrives on the Moon and hears the strange stories about people with rabbit ears...
He soon learns that he is a chosen warrior who
will save the Long-Ears race and their land Rabiluna from the evil Jadou Clan. With the magic gun, Daichi can summon Granzort – the Madou King of Earth – a giant robot.
There are also two other characters who summon robots in the way you described:
two other boys – Gus and Rabi. Gus receives a magic bow that summons Winzart – the Madou King of Wind, while Rabi receives a magic top that summons Aquabeat – the Madou King of Water.