The difference between WILL, BE GOING TO and the present continuous for future events

WILL and BE GOING TO for future intentions

Will (future simple) is used to express future intentions that are decided at the time of speaking (spontaneous offers, promises and decisions):

Come on, I'll help you with those bags. (seeing that someone is struggling with their shopping bags)

Be going to is used to express future intentions that have already been decided before the time of speaking:

I can't make it on Saturday. I'm going to help my parents around the house. (referring to a decision that the speaker has made in the past)

WILL and BE GOING TO for predictions

Will (future simple) is used to make predictions that are based on personal judgement, opinion or intuition:

I'm sure you'll have a lovely time in Italy. (opinion)

Be going to is used to make predictions that are based on present evidence. The predicted event is either very near (and can be seen) or seems sure to happen:

My sister's going to have a baby. (we can see that she is pregnant)

BE GOING TO and present continuous for future plans

Be going to refers to future intentions that have been decided but have not been fully planned:

We're going to get married. (referring to a past decision)

The present continuous tense refers to fixed future events and emphasises that plans or arrangements have already been made:

We are getting married next month. (the wedding has already been arranged)