According to o1 mini; while challenging, it's possible:

Carrying five NVIDIA RTX 3090 graphics cards in a standard backpack would be quite challenging for several reasons: 1. Physical Dimensions and Weight

Size:  Each RTX 3090 is a large GPU, typically around 12.3 inches (31.3 cm) in length, and it occupies two or even three expansion slots in a desktop setup. 
Weight:  Each card weighs approximately 2.35 pounds (1.07 kg). Five of them would total around 11.75 pounds (5.35 kg), not including any additional padding or protective materials you might need.

2. Backpack Capacity

Space:  Most standard backpacks, even larger ones designed for laptops, won't have the necessary compartmentalized space to securely fit five RTX 3090s. The cards are long and somewhat bulky, making them difficult to arrange without putting excessive pressure on each other.
Protection:  GPUs are sensitive electronic components. Without proper padding and secure placement, there's a high risk of damage from movement and impacts during transit.

3. Practical Considerations

Safety:  Carrying such valuable hardware in a backpack increases the risk of theft or accidental damage.
Heat and Ventilation:  While transporting, ensure that the GPUs are kept in a cool, dry environment to prevent any overheating, especially if they've been used recently.

Recommendations

Specialized Cases:  Invest in a hard-shell case specifically designed for transporting delicate electronics. These cases often have customizable foam inserts to securely hold and protect each component.
Fewer Units Per Trip:  If possible, carry fewer GPUs at a time to reduce risk and manage weight more effectively.
Shipping Services:  For transporting multiple high-value items like GPUs, consider using a reputable shipping service that offers secure, cushioned packaging and insurance.

Conclusion

While it might be physically possible to fit five RTX 3090s in a very large and sturdy backpack with adequate padding, it's not recommended due to the high risk of damage and the practical challenges involved. Using specialized transport solutions would be a safer and more effective approach.