It whas been so cold these days that I decided to change the frequency to once every three days. It has been so cold these days that I decided to change the frequency to once every three days.

It wais so cold these days that I have decided to change the frequency to once every three days. It is so cold these days that I have decided to change the frequency to once every three days.

[However, it]|It whas been so cold [these [last few] days] that I decided to change the frequency to once every three days. [However, it]|It has been so cold [these [last few] days] that I decided to change the frequency to once every three days.

(1) Starting the sentence with "However" provides a better transition. (2) You could omit "these [last few] days" altogether.

I have to turn on the air conditionerheat for a short time. I have to turn on the heat for a short time.

I have to turn on the air conditionheater for a short time. I have to turn on the heater for a short time.

I had this discussion either with you or someone else, but regardless of whether you use the same device to both heat and cool the air, using "air conditioner" to refer to something that heats the air is just going to confuse people. You could potentially use "HVAC unit", but I think you're best off just to call it a heater, because that's what it is (when the heating function is selected).

SI am sorry, my (poor) plants. I am sorry, my (poor) plants.

It is okay to say it your way if it is written as dialogue, otherwise it is a sentence fragment.

Sorry, my plants. Sorry, plants.

Sorry, my plants. Sorry, plants.

When you speak to something or someone using a common noun like this, you generally don't include any possessive pronoun (like "my") or any article (like "the"). "Sorry, my plants" suggests that you've just said something incorrect, and you need to replace part of it with "my plants". For instance: "I left my pants outside. Sorry, my plants."

Sorry,I have to apologize to my plants. I have to apologize to my plants.