1. Jeans are on sale, 30 % off today!

2. Jeans are 30% off today!
3. Jeans are for sale, 30 % off today!
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Are both the same in meaning? Can we erase 'on sale'? Can we say #3? What is the difference between #1 and #3?

2 answers

'on sale' means that the jeans are on discount, while 'for sale' means that you are selling jeans.
2. is possible.
For 3, 'are' is omitted in this kind of usage, so 'Jeans for sale, 30% off today!' Forms of 'be' are often used in informing, such as telling someone "These jeans are for sale" or in 1.

Hope this helps!
All mean about the same thing, yes.