What is the difference between a thobe, jubba and kurta?

Mostly the difference is region and cut, not religion. A thobe is a long robe common in the Arab world, a jubba is a loose, long outer garment, and a kurta is a South Asian tunic usually worn with trousers. None of them is a religious requirement. Names also overlap, so shops often use them loosely.

The thobe

The thobe (thawb in Arabic) is a long, one-piece robe with long sleeves, usually reaching the ankles. It is worn across the Arab world, and local names include dishdasha and kandura in the Gulf. Fabrics are light cotton for summer and thicker material for winter. Many converts like it because it is simple and covers the awrah well. See Can Muslim men wear a thobe in the West?

The jubba

Jubba (or jubbah) has been used in different ways. In classical Arabic, it means a long, wide outer garment, like a gown or coat, worn over other clothes. The Prophet ﷺ himself wore one: Al-Mughirah reported that "The Prophet wore a Roman Jubbah with tight sleeves" (Jami at-Tirmidhi 1768). Today, many Western shops use "jubba" for a thobe-style robe, sometimes with a looser or more decorated cut, so check the picture before you buy.

The kurta

A kurta is a South Asian tunic, usually loose and knee-length or longer, with side slits. It is worn with straight pyjama trousers or with a shalwar, and sometimes with jeans for a modern look. It comes in plain cotton for daily use and embroidered fabrics for weddings and Eid. See What is a shalwar kameez and do I have to wear one?

Which one is a religious requirement?

None. The Prophet ﷺ wore the clothes of his own time, and Muslims across the world wear their local dress. What Islam asks of men is a set of principles:

cover from the navel to the knees, in clothes that are not see-through or tight,
wear no silk or gold,
avoid copying women's clothing,
stay clean, and avoid pride and showing off.

Any of these garments can meet those principles, and so can jeans and a shirt.

On length: thobes are often cut to end at or just above the ankle. The Prophet ﷺ said, "The part of an izar which hangs below the ankles is in the Fire" (Sahih al-Bukhari 5787). Scholars differ on how to understand this. Many say it is a general rule to keep the garment above the ankles, while others link it to dragging clothes out of pride. See Do Muslim men have to wear trousers above the ankles?

How to choose

Climate: a thin cotton thobe or kurta for summer; wool or thicker fabrics for winter, with trousers or thermals underneath.
Occasion: a kurta is great for daily or casual wear, a thobe for Friday and Eid.
Fit: loose enough to pray comfortably and not see-through.
Budget: start with one plain, inexpensive piece. You do not need an expensive designer one.

Key points

Thobe, jubba and kurta come from different regions and styles.
None is a religious requirement.
The principles are modesty, cleanliness and no gold, silk or pride.
Choose by climate, comfort and occasion.

Tip: Try on a few styles before buying, ideally including a sujood test. Kneel and bend forward in the fitting room to see whether the garment stays comfortable and covered.