いらっしゃる
irassharu
Why one verb, いらっしゃる, swallows be, come and go
The picture A platform one step higher than your own floor, laid down for the other person: everything they do — sitting, arriving, walking — is drawn only on that platform. いらっしゃる raises the other person's be/come/go one step; おる and 参る lower your own by one step.
Where the picture shows up in いらっしゃる
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resting on the platform
社長は今、席にいらっしゃいます。
shachō wa ima, seki ni irasshaimasu
The president is at his desk right now.
The president's chair standing on a platform one step up, with him seated on it.
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stepping up onto it
お客様が二名いらっしゃいました。
o-kyakusama ga nimei irasshaimashita
Two customers have arrived.
The shop door opening and two customers stepping up onto the raised floor as they come in.
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the platform laid into tomorrow
明日は何時にいらっしゃいますか。
asu wa nanji ni irasshaimasu ka
What time will you be coming tomorrow?
A raised walkway laid across tomorrow's calendar box, with only the visitor's footsteps drawn on it.
Whether they are coming to you or going elsewhere, the platform stays at the same height.
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offering the step at the door
いらっしゃいませ、こちらへどうぞ。
irasshaimase, kochira e dōzo
Welcome — right this way, please.
Holding the door open with a bow, pushing the step stool out toward the guest.
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a platform laid far away
ご両親は大阪にいらっしゃるんですか。
go-ryōshin wa Ōsaka ni irassharu n desu ka
Do your parents live in Osaka?
On a map, a raised chair set down on Osaka with the other person's parents seated in it.
All of it in one picture
いらっしゃる isn't three verbs fused together; it is one raised floor. Someone resting on that platform is "is present"; someone stepping up onto it is "has arrived"; the platform continuing into tomorrow is "will be coming". Holding the door and sliding the step toward a customer is いらっしゃいませ, and a platform laid all the way out to Osaka means "they live there". The height of the step never changes.