Covering 3 Months' Time of Story

Sure! In one of my books I summed up three years of elapsed time in a few paragraphs. You do it by breaking that old familiar rule...you tell instead of show.

"By the time three months had passed, Hero was thinner, stronger, and considerably better with the sword. He had several new scars to prove he hadn't been idle."

Then you return to the showing. Telling is great for getting past time jumps.

Libby is right on...

He put the six pack beer on the counter when someone tapped his shoulder. He turned and the bartender from the Neon Hooter club stood there smiling at him. He hadn't seen her in three months, though he had dreamed of her often while training in the desert.

Or something like that...

As the first poster said, just tell the reader how much time has passed and then go back to your normal style of writing. In the novel I am trying to write right now, I have to keep jumping over days of nothing but travel from one place to another, and I use lines such as 'It had taken several days but the group finally reached the edge of the lake, their coats covered in icicles from the wintry landscape they had just left behind.'

In your case I would use lines such as 'Three months, and he was only finally beginning to get a grasp on what they were trying to teach him.' If you do actually want to spread it across a chapter as well, you could probably show how boring the routine is; eat, train, sleep, eat, train, sleep so on and so on, or show the competition between him and a rival/friend who has also signed up for training, there are plenty of things you could show about the three months he is there. I hope that made sense

In The Magician by Raymond E Feist he skipped four years. I cant remember exactly how he did it as I havent read it for years, I dont think he literally did it between chapters though.

I'll explain more about the example. Major spoiler alert...

...

There are two protags in that book, Pug and Tomas. Plot is, a race from another world invade. Pug is captured in the war and taken to the invaders homeworld where he slaves away for four years. The way Fiest got away with skipping forward so long though, if I remember right, is because the situation for both MCs was pretty static in that four years; Pug was a slave, Tomas was becoming a soldier fighting the invaders.

I think this is a good example, because its fairly extreme. FOUR YEARS had been skipped, yet the characters had adapted while remaining the same people.

So skipping three months can be done. But, if you literally need it between chapters I duno how you would go about it. If you have more PoV characters and are portraying some events within that three month period you could just drop MC off where they will train, then switch PoV and show that the world isnt on pause, then go back to MC when theyve finished.

Avoid showing anything that is boring. If its boring, then reading about it is going to be boring in most cases...

I just skipped from a wedding to 10 years later when the MC is still happily married and has two children.

I would just chapter break and at the beginning of the next chapter mention that he's been training for three months. Readers aren't dumb. They can keep up with logical time skips.

You might even just start three months in the future at the start of a new chapter and subtly show an advance in time. Like someone else said, avoid anything boring. Don't be afraid to try a few things. Readers are smart; they'll follow what you're doing, so long as you don't purposefully leave them behind.

In short--paraphrase. Become comfortable with transitions. It can be done. A book isn't a day-by-day tell all. It's a train that has multiple stops on a long journey. Don't be afraid to travel a little bit before stopping.