font-size: 3vw; means that the font size will be 3% of the viewport width. So when the viewport width is 1200px - the font size will be 3% * 1200px = 36px.

So a max-font-size of 36px can be easily implemented using a single media query to override the default 3vw font-size value.

Codepen demo (Resize Browser)

div {
  font-size: 3vw;
}
@media screen and (min-width: 1200px) {
  div {
     font-size: 36px;
  }
}

<div>hello</div>

Update: With the new CSS min() function, we can simplify the above code - without using media queries (caniuse)

div {
  font-size: min(3vw, 36px);
}

In the above example, the font-size will be at most 36px, but will decrease to 3vw if the the viewport is less than 1200px wide (where 3vw computes to a value less than 36px )

That being said, using viewport units for font-size in the above way is problematic because when the viewport width is much smaller - say 320px - then the rendered font size will become 0.03 x 320 = 9.6px which is very (too) small.

In order to overcome this problem, I can recommend using a technique called Fluid Type AKA CSS Locks.

A CSS lock is a specific kind of CSS value calculation where:

there is a minimum value and a maximum value,

and two breakpoints (usually based on the viewport width),

and between those breakpoints, the actual value goes linearly from the minimum to the maximum.

So let's say we want to apply the above technique such that the minimum font-size is 16px at a viewport width of 600px or less, and will increase linearly until it reaches a maximum of 32px at a viewport width of 1200px.

This can be represented as follows (see this CSS-tricks article for more details):

div {
  font-size: 16px;
}
@media screen and (min-width: 600px) {
  div {
    font-size: calc(16px + 16 * ((100vw - 600px) / 600));
  }
}
@media screen and (min-width: 1200px) {
  div {
    font-size: 32px;
  }
}

Alternatively, we could use this SASS mixin which does all of the math for us so that the CSS would look something like this:

/* 
     1) Set a min-font-size of 16px when viewport width < 600px
     2) Set a max-font-size of 32px when viewport width > 1200px and
     3) linearly increase the font-size from 16->32px 
     between a viewport width of 600px-> 1200px 
*/

div {
  @include fluid-type(font-size, 600px, 1200px, 16px, 32px);
}

// ----
// libsass (v3.3.6)
// ----

// =========================================================================
//
//  PRECISE CONTROL OVER RESPONSIVE TYPOGRAPHY FOR SASS
//  ---------------------------------------------------
//  Indrek Paas @indrekpaas
//
//  Inspired by Mike Riethmuller's Precise control over responsive typography
//                                                                         
//
//  `strip-unit()` function by Hugo Giraudel
//  
//  11.08.2016 Remove redundant `&` self-reference
//  31.03.2016 Remove redundant parenthesis from output
//  02.10.2015 Add support for multiple properties
//  24.04.2015 Initial release
//
// =========================================================================

@function strip-unit($value) {
  @return $value / ($value * 0 + 1);
}

@mixin fluid-type($properties, $min-vw, $max-vw, $min-value, $max-value) {
  @each $property in $properties {
    #{$property}: $min-value;
  }

  @media screen and (min-width: $min-vw) {
    @each $property in $properties {
      #{$property}: calc(#{$min-value} + #{strip-unit($max-value - $min-value)} * (100vw - #{$min-vw}) / #{strip-unit($max-vw - $min-vw)});
    }
  }

  @media screen and (min-width: $max-vw) {
    @each $property in $properties {
      #{$property}: $max-value;
    }
  }
}

// Usage:
// ======

// /* Single property */
// html {
//   @include fluid-type(font-size, 320px, 1366px, 14px, 18px);
// }

// /* Multiple properties with same values */
// h1 {
//   @include fluid-type(padding-bottom padding-top, 20em, 70em, 2em, 4em);
// }

////////////////////////////////////////////////////////////////////////////

div {
  @include fluid-type(font-size, 600px, 1200px, 16px, 32px);
}
@media screen and (max-width: 600px) {
  div {
     font-size: 16px;
  }
}
@media screen and (min-width: 1200px) {
  div {
     font-size: 36px;
  }
}

<div>Responsive Typography technique known as Fluid Type or CSS Locks. 
  Resize the browser window to see the effect.
</div>

Codepen Demo

Update: We can use the new clamp() CSS function (caniuse) to refactor the above code to simply:

div {
  font-size: clamp(16px, 3vw, 32px);
}

see MDN:

clamp() allows you to set a font-size that grows with the size of the viewport, but doesn't go below a minimum font-size or above a maximum font-size. It has the same effect as the code in Fluid Typography but in one line, and without the use of media queries.

p { font-size: clamp(1rem, 2.5vw, 1.5rem); }
<p>
If 2.5vw is less than 1rem, the font-size will be 1rem.
If 2.5vw is greater than 1.5rem, the font-size will be 1.5rem.
Otherwise, it will be 2.5vw.
</p>

How to implement max-font-size?

Codepen demo (Resize Browser)

Update: With the new CSS min() function, we can simplify the above code - without using media queries (caniuse)

Update: We can use the new clamp() CSS function (caniuse) to refactor the above code to simply:

Further Reading