Decrease:

• Can be a transitive or intransitive verb
• Can be a noun
• Not used as an adjective

Lower:

• Can be a transitive verb but has very rare intransitive use cases
• Not used as a noun
• Can be an adjective

Reduce:

• Can be a transitive verb only
• Not used as a noun
• Not used as an adjective

Out of these, "reduce" is probably the most commonly best choice for transitive verb usage and "decrease" is almost always the best choice for intransitive verb usage.

But I would recommend first scanning your vocabulary to determine whether there exists a better intransitive verb than "decrease," as sometimes it can sound clunky when used in intransitive form. For example:

The temperature decreased.

This is technically acceptable, but often there are better intransitive verb choices available:

The temperature dropped.

The temperature fell.

The temperature went down.

Another example:

Grandma's cognitive skills decreased as she aged.

Again, technically fine, but it just sounds smoother to express it differently:

Grandma's cognitive skills declined as she aged.

Grandma's cognitive skills diminished as she aged.

To recap, "decrease" is acceptable in the above examples—but if you can find a verb that equally or better expresses the same concept you are trying to convey, use it instead. This is a totally minor style recommendation and likely will not impact your everyday communication.

EDIT: Examples by request

By request in the comments, here are some examples of how the three verbs can and cannot be used:

"Lower"

Please lower the window curtains.

Please decrease the window curtains.

Please reduce the window curtains.

The fundamental reason why only "lower" is an acceptable verb to describe adjusting the position of the window curtains is that only "lower" can be used to describe adjusting the actual position of an item in three-dimensional space, so as to cause it to be closer to the earth's core.

Lower can also be used for other physical characteristics, such as sound and light levels. "Please lower the lights" or "please lower the TV volume" are completely natural-sounding and acceptable.

"Reduce [to]"

The fire reduced the building to a pile of rubble.

The fire decreased the building to a pile of rubble.

The fire lowered the building to a pile of rubble.

The heavy rainstorm reduced road visibility to little more than a blur.

The heavy rainstorm reduced road visibility.

The heavy rainstorm decreased road visibility to little more than a blur.

The heavy rainstorm decreased road visibility.

The heavy rainstorm lowered road visibility to little more than a blur.

The heavy rainstorm lowered road visibility.

These examples show that "reduce" can be combined with "to" to describe the actual conversion of one thing into some other thing (e.g. "building" becomes "pile of rubble"), the latter being inherently different and of lesser quality or quantity than the former.

But when you're merely dealing with some quantifiable characteristic (e.g. "road visibility"), all three verbs are acceptable.

A few advanced examples:

The professor's harsh words reduced the student to tears.

The performance review reduced the CEO to nothing but a pretty face.

I will not be reduced to perform a janitor's duties.

In these examples, "reduce [to]" is used to describe an attack or offense against an actual person. What follows "to" can be an undesirable state of being, an exaggerated and offensive description of the person being "reduced," or an undesirable activity in which the "reduced" person becomes engaged. The implication with this last possibility is that the person is "too good" (e.g. wealthy, intelligent, beautiful, entitled) to be forced to engage in such an activity.

If my words reduce you to tears, then I've made you cry (i.e. you are now in a state of tears).

If my review of you reduces you to nothing but a pretty face, then I've expressed my opinion that you are brainless and that your only redeeming quality is your physical beauty (i.e. an exaggerated and offense description of you).

If I reduce you to perform a janitor's duties, I've made you clean up after yourself (and maybe other people), despite the fact that you have six advanced degrees from Harvard, Yale, and Stanford (e.g. you're way too smart and accomplished for me to force you to perform cleaning duties).

The common thing between the two is they describe a nonphysical part of a person. Now lets define each to get the difference.

Usually when you say "spirit" you are referring to the attitude of a person. It describes emotional things like motivation, courage, enthusiasm, etc.

"Mind" on the other hand describes the intellect and intelligence. The part of a person that has sense and logic and is not driven by emotions. The part that processes and memorizes things.